[flutter_tools] Null safe update packages (#99357)

packages/flutter_tools/lib/src/commands/update_packages.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'dart:collection';
 
 import 'package:meta/meta.dart';
@@ -46,14 +44,12 @@ class UpdatePackagesCommand extends FlutterCommand {
         'force-upgrade',
         help: 'Attempt to update all the dependencies to their latest versions.\n'
               'This will actually modify the pubspec.yaml files in your checkout.',
-        defaultsTo: false,
         negatable: false,
       )
       ..addFlag(
         'paths',
         help: 'Finds paths in the dependency chain leading from package specified '
               'in "--from" to package specified in "--to".',
-        defaultsTo: false,
         negatable: false,
       )
       ..addOption(
@@ -70,7 +66,6 @@ class UpdatePackagesCommand extends FlutterCommand {
         'transitive-closure',
         help: 'Prints the dependency graph that is the transitive closure of '
               'packages the Flutter SDK depends on.',
-        defaultsTo: false,
         negatable: false,
       )
       ..addFlag(
@@ -78,25 +73,21 @@ class UpdatePackagesCommand extends FlutterCommand {
         help: 'Only prints the dependency graph that is the transitive closure '
               'that a consumer of the Flutter SDK will observe (when combined '
               'with transitive-closure).',
-        defaultsTo: false,
         negatable: false,
       )
       ..addFlag(
         'verify-only',
         help: 'Verifies the package checksum without changing or updating deps.',
-        defaultsTo: false,
         negatable: false,
       )
       ..addFlag(
         'offline',
         help: 'Use cached packages instead of accessing the network.',
-        defaultsTo: false,
         negatable: false,
       )
       ..addFlag(
         'crash',
         help: 'For Flutter CLI testing only, forces this command to throw an unhandled exception.',
-        defaultsTo: false,
         negatable: false,
       )
       ..addOption(
@@ -121,8 +112,7 @@ class UpdatePackagesCommand extends FlutterCommand {
 
 
   // Lazy-initialize the net utilities with values from the context.
-  Net _cachedNet;
-  Net get _net => _cachedNet ??= Net(
+  late final Net _net = Net(
     httpClientFactory: context.get<HttpClientFactory>(),
     logger: globals.logger,
     platform: globals.platform,
@@ -131,9 +121,15 @@ class UpdatePackagesCommand extends FlutterCommand {
   Future<void> _downloadCoverageData() async {
     final String urlBase = globals.platform.environment['FLUTTER_STORAGE_BASE_URL'] ?? 'https://storage.googleapis.com';
     final Uri coverageUri = Uri.parse('$urlBase/flutter_infra_release/flutter/coverage/lcov.info');
-    final List<int> data = await _net.fetchUrl(coverageUri);
+    final List<int>? data = await _net.fetchUrl(
+      coverageUri,
+      maxAttempts: 3,
+    );
+    if (data == null) {
+      throwToolExit('Failed to fetch coverage data from $coverageUri');
+    }
     final String coverageDir = globals.fs.path.join(
-      Cache.flutterRoot,
+      Cache.flutterRoot!,
       'packages/flutter/coverage',
     );
     globals.fs.file(globals.fs.path.join(coverageDir, 'lcov.base.info'))
@@ -146,7 +142,7 @@ class UpdatePackagesCommand extends FlutterCommand {
 
   @override
   Future<FlutterCommandResult> runCommand() async {
-    final List<Directory> packages = runner.getRepoPackages();
+    final List<Directory> packages = runner!.getRepoPackages();
 
     final bool forceUpgrade = boolArg('force-upgrade');
     final bool isPrintPaths = boolArg('paths');
@@ -268,7 +264,7 @@ class UpdatePackagesCommand extends FlutterCommand {
           checksumDependencies[data.name] = data.version;
         }
       }
-      final String checksum = _computeChecksum(checksumDependencies.keys, (String name) => checksumDependencies[name]);
+      final String checksum = _computeChecksum(checksumDependencies.keys, (String name) => checksumDependencies[name]!);
       if (checksum != pubspec.checksum.value) {
         // If the checksum doesn't match, they may have added or removed some dependencies.
         // we need to run update-packages to recapture the transitive deps.
@@ -294,12 +290,12 @@ class UpdatePackagesCommand extends FlutterCommand {
   }
 
   void _collectDependencies({
-    @required List<Directory> packages,
-    @required List<PubspecYaml> pubspecs,
-    @required Set<String> specialDependencies,
-    @required Map<String, PubspecDependency> explicitDependencies,
-    @required Map<String, PubspecDependency> allDependencies,
-    @required bool doUpgrade,
+    required List<Directory> packages,
+    required List<PubspecYaml> pubspecs,
+    required Set<String> specialDependencies,
+    required Map<String, PubspecDependency> explicitDependencies,
+    required Map<String, PubspecDependency> allDependencies,
+    required bool doUpgrade,
   }) {
     // Visit all the directories with pubspec.yamls we care about.
     for (final Directory directory in packages) {
@@ -320,12 +316,12 @@ class UpdatePackagesCommand extends FlutterCommand {
           // This makes sure that we don't import a package in two different
           // ways, e.g. by saying "sdk: flutter" in one pubspec.yaml and
           // saying "path: ../../..." in another.
-          final PubspecDependency previous = allDependencies[dependency.name];
-          if (dependency.kind != previous.kind || dependency.lockTarget != previous.lockTarget) {
+          final PubspecDependency previous = allDependencies[dependency.name]!;
+          if (dependency.kind != previous.kind || dependency._lockTarget != previous._lockTarget) {
             throwToolExit(
                 'Inconsistent requirements around ${dependency.name}; '
-                    'saw ${dependency.kind} (${dependency.lockTarget}) in "${dependency.sourcePath}" '
-                    'and ${previous.kind} (${previous.lockTarget}) in "${previous.sourcePath}".'
+                    'saw ${dependency.kind} (${dependency._lockTarget}) in "${dependency.sourcePath}" '
+                    'and ${previous.kind} (${previous._lockTarget}) in "${previous.sourcePath}".'
             );
           }
           if (dependency.version != previous.version) {
@@ -350,12 +346,12 @@ class UpdatePackagesCommand extends FlutterCommand {
           // This makes sure that we don't import a package in two different
           // ways, e.g. by saying "sdk: flutter" in one pubspec.yaml and
           // saying "path: ../../..." in another.
-          final PubspecDependency previous = explicitDependencies[dependency.name];
-          if (dependency.kind != previous.kind || dependency.lockTarget != previous.lockTarget) {
+          final PubspecDependency previous = explicitDependencies[dependency.name]!;
+          if (dependency.kind != previous.kind || dependency._lockTarget != previous._lockTarget) {
             throwToolExit(
                 'Inconsistent requirements around ${dependency.name}; '
-                'saw ${dependency.kind} (${dependency.lockTarget}) in "${dependency.sourcePath}" '
-                'and ${previous.kind} (${previous.lockTarget}) in "${previous.sourcePath}".'
+                'saw ${dependency.kind} (${dependency._lockTarget}) in "${dependency.sourcePath}" '
+                'and ${previous.kind} (${previous._lockTarget}) in "${previous.sourcePath}".'
             );
           }
         }
@@ -373,11 +369,11 @@ class UpdatePackagesCommand extends FlutterCommand {
   }
 
   Future<void> _generateFakePackage({
-    Directory tempDir,
-    Iterable<PubspecDependency> dependencies,
-    List<PubspecYaml> pubspecs,
-    PubDependencyTree tree,
-    bool doUpgrade,
+    required Directory tempDir,
+    required Iterable<PubspecDependency> dependencies,
+    required List<PubspecYaml> pubspecs,
+    required PubDependencyTree tree,
+    required bool doUpgrade,
   }) async {
     try {
       final File fakePackage = _pubspecFor(tempDir);
@@ -390,7 +386,7 @@ class UpdatePackagesCommand extends FlutterCommand {
       );
       // Create a synthetic flutter SDK so that transitive flutter SDK
       // constraints are not affected by this upgrade.
-      Directory temporaryFlutterSdk;
+      Directory? temporaryFlutterSdk;
       if (doUpgrade) {
         temporaryFlutterSdk = createTemporaryFlutterSdk(
           globals.logger,
@@ -407,8 +403,7 @@ class UpdatePackagesCommand extends FlutterCommand {
         directory: tempDir.path,
         upgrade: doUpgrade,
         offline: boolArg('offline'),
-        flutterRootOverride: doUpgrade ? temporaryFlutterSdk.path : null,
-        generateSyntheticPackage: false,
+        flutterRootOverride: temporaryFlutterSdk?.path,
       );
       // Cleanup the temporary SDK
       try {
@@ -437,10 +432,10 @@ class UpdatePackagesCommand extends FlutterCommand {
   }
 
   bool _upgradePubspecs({
-    @required PubDependencyTree tree,
-    @required List<PubspecYaml> pubspecs,
-    @required Set<String> specialDependencies,
-    @required Map<String, PubspecDependency> explicitDependencies,
+    required PubDependencyTree tree,
+    required List<PubspecYaml> pubspecs,
+    required Set<String> specialDependencies,
+    required Map<String, PubspecDependency> explicitDependencies,
   }) {
     // The transitive dependency tree for the fake package does not contain
     // dependencies between Flutter SDK packages and pub packages. We add them
@@ -453,7 +448,7 @@ class UpdatePackagesCommand extends FlutterCommand {
       tree._dependencyTree[package] = <String>{};
       for (final PubspecDependency dependency in pubspec.dependencies) {
         if (dependency.kind == DependencyKind.normal) {
-          tree._dependencyTree[package].add(dependency.name);
+          tree._dependencyTree[package]!.add(dependency.name);
         }
       }
     }
@@ -466,7 +461,7 @@ class UpdatePackagesCommand extends FlutterCommand {
     }
 
     if (boolArg('paths')) {
-      showDependencyPaths(from: stringArg('from'), to: stringArg('to'), tree: tree);
+      showDependencyPaths(from: stringArg('from')!, to: stringArg('to')!, tree: tree);
       return true;
     }
 
@@ -499,8 +494,8 @@ class UpdatePackagesCommand extends FlutterCommand {
       'Running "flutter pub get" in affected packages...',
     );
     try {
-      // int.tryParse will convert an empty string to null
-      final int/*?*/ maxJobs = int.tryParse(stringArg('jobs') ?? '');
+      // int.tryParse will not accept null, but will convert empty string to null
+      final int? maxJobs = int.tryParse(stringArg('jobs') ?? '');
       final TaskQueue<void> queue = TaskQueue<void>(maxJobs: maxJobs);
       for (final Directory dir in packages) {
         unawaited(queue.add(() async {
@@ -512,7 +507,6 @@ class UpdatePackagesCommand extends FlutterCommand {
             // All dependencies should already have been downloaded by the fake
             // package, so the concurrent checks can all happen offline.
             offline: true,
-            generateSyntheticPackage: false,
             printProgress: false,
           );
           stopwatch.stop();
@@ -530,10 +524,10 @@ class UpdatePackagesCommand extends FlutterCommand {
         globals.printStatus('Downloaded lcov data for package:flutter in ${seconds.toStringAsFixed(1)}s...');
       }));
       await queue.tasksComplete;
-      status?.stop();
+      status.stop();
       // The exception is rethrown, so don't catch only Exceptions.
     } catch (exception) { // ignore: avoid_catches_without_on_clauses
-      status?.cancel();
+      status.cancel();
       rethrow;
     }
 
@@ -542,9 +536,9 @@ class UpdatePackagesCommand extends FlutterCommand {
   }
 
   void showDependencyPaths({
-    @required String from,
-    @required String to,
-    @required PubDependencyTree tree,
+    required String from,
+    required String to,
+    required PubDependencyTree tree,
   }) {
     if (!tree.contains(from)) {
       throwToolExit('Package $from not found in the dependency tree.');
@@ -564,9 +558,9 @@ class UpdatePackagesCommand extends FlutterCommand {
         paths.add(link);
       }
       if (link.from != null) {
-        visited.add(link.from.to);
+        visited.add(link.from!.to);
       }
-      for (final String dependency in tree._dependencyTree[link.to]) {
+      for (final String dependency in tree._dependencyTree[link.to]!) {
         if (!visited.contains(dependency)) {
           traversalQueue.addFirst(_DependencyLink(from: link, to: dependency));
         }
@@ -577,7 +571,7 @@ class UpdatePackagesCommand extends FlutterCommand {
       final StringBuffer buf = StringBuffer();
       while (path != null) {
         buf.write(path.to);
-        path = path.from;
+        path = path.from!;
         if (path != null) {
           buf.write(' <- ');
         }
@@ -593,11 +587,11 @@ class UpdatePackagesCommand extends FlutterCommand {
 
 class _DependencyLink {
   _DependencyLink({
-    @required this.from,
-    @required this.to,
+    required this.from,
+    required this.to,
   });
 
-  final _DependencyLink from;
+  final _DependencyLink? from;
   final String to;
 
   @override
@@ -662,7 +656,7 @@ class PubspecYaml {
   final String name;
 
   /// The package version.
-  final String version;
+  final String? version;
 
   final List<PubspecLine> inputData; // Each line of the pubspec.yaml file, parsed(ish).
 
@@ -679,9 +673,9 @@ class PubspecYaml {
   /// about (since they're all under our control).
   static PubspecYaml _parse(File file, List<String> lines) {
     final String filename = file.path;
-    String packageName;
-    String packageVersion;
-    PubspecChecksum checksum; // the checksum value used to verify that dependencies haven't changed.
+    String? packageName;
+    String? packageVersion;
+    PubspecChecksum? checksum; // the checksum value used to verify that dependencies haven't changed.
     final List<PubspecLine> result = <PubspecLine>[]; // The output buffer.
     Section section = Section.other; // Which section we're currently reading from.
     bool seenMain = false; // Whether we've seen the "dependencies:" section.
@@ -695,13 +689,13 @@ class PubspecYaml {
     // whatnot) have the style of having extra data after the line that declares
     // the dependency. So we track what is the "current" (or "last") dependency
     // that we are dealing with using this variable.
-    PubspecDependency lastDependency;
+    PubspecDependency? lastDependency;
     for (int index = 0; index < lines.length; index += 1) {
       String line = lines[index];
       if (lastDependency == null) {
         // First we look to see if we're transitioning to a new top-level section.
         // The PubspecHeader.parse static method can recognize those headers.
-        final PubspecHeader header = PubspecHeader.parse(line); // See if it's a header.
+        final PubspecHeader? header = PubspecHeader.parse(line); // See if it's a header.
         if (header != null) { // It is!
           section = header.section; // The parser determined what kind of section it is.
           if (section == Section.header) {
@@ -748,7 +742,7 @@ class PubspecYaml {
           }
         } else {
           // We're in a section we care about. Try to parse out the dependency:
-          final PubspecDependency dependency = PubspecDependency.parse(line, filename: filename);
+          final PubspecDependency? dependency = PubspecDependency.parse(line, filename: filename);
           if (dependency != null) { // We got one!
             // Track whether or not this a dev dependency.
             dependency.isDevDependency = seenDev;
@@ -813,7 +807,7 @@ class PubspecYaml {
               throw StateError('Invalid pubspec.yaml: a "git" dependency section terminated early.');
             }
             line = lines[index];
-            lastDependency._lockLine += '\n$line';
+            lastDependency._lockLine = '${lastDependency._lockLine}\n$line';
           } while (line.startsWith('   '));
         }
         // We're done with this special dependency, so reset back to null so
@@ -821,7 +815,7 @@ class PubspecYaml {
         lastDependency = null;
       }
     }
-    return PubspecYaml._(file, packageName, packageVersion, result, checksum ?? PubspecChecksum(null, ''));
+    return PubspecYaml._(file, packageName!, packageVersion, result, checksum ?? PubspecChecksum(null, ''));
   }
 
   /// This returns all the explicit dependencies that this pubspec.yaml lists under dependencies.
@@ -860,9 +854,9 @@ class PubspecYaml {
     Section section = Section.other; // the section we're currently handling
 
     // the line number where we're going to insert the transitive dependencies.
-    int endOfDirectDependencies;
+    int? endOfDirectDependencies;
     // The line number where we're going to insert the transitive dev dependencies.
-    int endOfDevDependencies;
+    int? endOfDevDependencies;
     // Walk the pre-parsed input file, outputting it unmodified except for
     // updating version numbers, removing the old transitive dependencies lines,
     // and adding our new transitive dependencies lines. We also do a little
@@ -913,7 +907,7 @@ class PubspecYaml {
                 // flutter" line) then we output that too.
                 output.add(data.line);
                 if (data.lockLine != null) {
-                  output.add(data.lockLine);
+                  output.add(data.lockLine!);
                 }
               }
               // Remember that we've dealt with this dependency so we don't
@@ -940,7 +934,7 @@ class PubspecYaml {
             // In other sections, pass everything through in its original form.
             output.add(data.line);
             if (data.lockLine != null) {
-              output.add(data.lockLine);
+              output.add(data.lockLine!);
             }
             break;
         }
@@ -1040,7 +1034,7 @@ class PubspecYaml {
       line = line.trimRight();
       if (line == '') {
         if (!hadBlankLine) {
-          contents.writeln('');
+          contents.writeln();
         }
         hadBlankLine = true;
       } else {
@@ -1070,7 +1064,7 @@ class PubspecChecksum extends PubspecLine {
   /// and special dependencies sorted lexically.
   ///
   /// If the line cannot be parsed, [value] will be null.
-  final String value;
+  final String? value;
 
   /// Parses a [PubspecChecksum] from a line.
   ///
@@ -1087,7 +1081,12 @@ class PubspecChecksum extends PubspecLine {
 
 /// A header, e.g. "dependencies:".
 class PubspecHeader extends PubspecLine {
-  PubspecHeader(String line, this.section, { this.name, this.value }) : super(line);
+  PubspecHeader(
+    String line,
+    this.section, {
+    this.name,
+    this.value,
+  }) : super(line);
 
   /// The section of the pubspec where the parse [line] appears.
   final Section section;
@@ -1102,7 +1101,7 @@ class PubspecHeader extends PubspecLine {
   /// ```
   /// version: 0.16.5
   /// ```
-  final String name;
+  final String? name;
 
   /// The value in the pubspec line providing a name/value pair, such as "name"
   /// and "version".
@@ -1114,9 +1113,9 @@ class PubspecHeader extends PubspecLine {
   /// ```
   /// version: 0.16.5
   /// ```
-  final String value;
+  final String? value;
 
-  static PubspecHeader parse(String line) {
+  static PubspecHeader? parse(String line) {
     // We recognize any line that:
     //  * doesn't start with a space (i.e. is aligned on the left edge)
     //  * ignoring trailing spaces and comments, ends with a colon
@@ -1167,14 +1166,14 @@ class PubspecDependency extends PubspecLine {
     String line,
     this.name,
     this.suffix, {
-    @required this.isTransitive,
-    DependencyKind kind,
-    this.version,
-    this.sourcePath,
+    required this.isTransitive,
+    required DependencyKind kind,
+    required this.version,
+    required this.sourcePath,
   }) : _kind = kind,
        super(line);
 
-  static PubspecDependency parse(String line, { @required String filename }) {
+  static PubspecDependency? parse(String line, { required String filename }) {
     // We recognize any line that:
     //  * starts with exactly two spaces, no more or less
     //  * has some content, then a colon
@@ -1231,26 +1230,25 @@ class PubspecDependency extends PubspecLine {
   final String version; // the version string if found, or blank.
   final bool isTransitive; // whether the suffix matched kTransitiveMagicString
   final String sourcePath; // the filename of the pubspec.yaml file, for error messages
-  bool isDevDependency; // Whether this dependency is under the `dev dependencies` section.
+  late bool isDevDependency; // Whether this dependency is under the `dev dependencies` section.
 
   DependencyKind get kind => _kind;
   DependencyKind _kind = DependencyKind.normal;
 
   /// If we're a path or sdk dependency, the path or sdk in question.
-  String get lockTarget => _lockTarget;
-  String _lockTarget;
+  String? _lockTarget;
 
   /// If we were a two-line dependency, the second line (see the inherited [line]
   /// for the first).
-  String get lockLine => _lockLine;
-  String _lockLine;
+  String? get lockLine => _lockLine;
+  String? _lockLine;
 
   /// If we're a path or sdk dependency, whether we were found in a
   /// dependencies/dev_dependencies section, or a dependency_overrides section.
   /// We track this so that we can put ourselves in the right section when
   /// generating the fake pubspec.yaml.
   bool get lockIsOverride => _lockIsOverride;
-  bool _lockIsOverride;
+  late bool _lockIsOverride;
 
   static const String _pathPrefix = '    path: ';
   static const String _sdkPrefix = '    sdk: ';
@@ -1268,9 +1266,10 @@ class PubspecDependency extends PubspecLine {
       return true;
     }
 
-    if (_kind == DependencyKind.path &&
-        !globals.fs.path.isWithin(globals.fs.path.join(Cache.flutterRoot, 'bin'), _lockTarget) &&
-        globals.fs.path.isWithin(Cache.flutterRoot, _lockTarget)) {
+    final String? lockTarget = _lockTarget;
+    if (_kind == DependencyKind.path && lockTarget != null &&
+        !globals.fs.path.isWithin(globals.fs.path.join(Cache.flutterRoot!, 'bin'), lockTarget) &&
+        globals.fs.path.isWithin(Cache.flutterRoot!, lockTarget)) {
       return true;
     }
 
@@ -1281,7 +1280,7 @@ class PubspecDependency extends PubspecLine {
   /// We throw if we couldn't parse this line.
   /// We return true if we parsed it and stored the line in lockLine.
   /// We return false if we parsed it and it's a git dependency that needs the next few lines.
-  bool parseLock(String line, String pubspecPath, { @required bool lockIsOverride }) {
+  bool parseLock(String line, String pubspecPath, { required bool lockIsOverride }) {
     assert(lockIsOverride != null);
     assert(kind == DependencyKind.unknown);
     if (line.startsWith(_pathPrefix)) {
@@ -1333,27 +1332,27 @@ class PubspecDependency extends PubspecLine {
         }
         break;
       case DependencyKind.path:
-        if (_lockIsOverride) {
+        if (lockIsOverride) {
           dependencies.writeln('  $name: $versionToUse');
           overrides.writeln('  $name:');
-          overrides.writeln('    path: $lockTarget');
+          overrides.writeln('    path: $_lockTarget');
         } else {
           dependencies.writeln('  $name:');
-          dependencies.writeln('    path: $lockTarget');
+          dependencies.writeln('    path: $_lockTarget');
         }
         break;
       case DependencyKind.sdk:
-        if (_lockIsOverride) {
+        if (lockIsOverride) {
           dependencies.writeln('  $name: $versionToUse');
           overrides.writeln('  $name:');
-          overrides.writeln('    sdk: $lockTarget');
+          overrides.writeln('    sdk: $_lockTarget');
         } else {
           dependencies.writeln('  $name:');
-          dependencies.writeln('    sdk: $lockTarget');
+          dependencies.writeln('    sdk: $_lockTarget');
         }
         break;
       case DependencyKind.git:
-        if (_lockIsOverride) {
+        if (lockIsOverride) {
           dependencies.writeln('  $name: $versionToUse');
           overrides.writeln('  $name:');
           overrides.writeln(lockLine);
@@ -1398,20 +1397,19 @@ String _generateFakePubspec(
       for (final PubspecDependency dependency in dependencies)
         dependency.name,
     };
-    for (final String package in kManuallyPinnedDependencies.keys) {
+    kManuallyPinnedDependencies.forEach((String package, String version) {
       // Don't add pinned dependency if it is not in the set of all transitive dependencies.
       if (!allTransitive.contains(package)) {
         if (verbose) {
           globals.printStatus('Skipping $package because it was not transitive');
         }
-        continue;
+        return;
       }
-      final String version = kManuallyPinnedDependencies[package];
       result.writeln('  $package: $version');
       if (verbose) {
         globals.printStatus('  - $package: $version');
       }
-    }
+    });
   }
   for (final PubspecDependency dependency in dependencies) {
     if (!dependency.pointsToSdk) {
@@ -1427,7 +1425,7 @@ String _generateFakePubspec(
 /// It ends up holding the full graph of dependencies, and the version number for
 /// each one.
 class PubDependencyTree {
-  final Map<String, String> _versions = <String, String>{};
+  final Map<String, String?> _versions = <String, String>{};
   final Map<String, Set<String>> _dependencyTree = <String, Set<String>>{};
 
   /// Handles the output from "pub deps --style=compact".
@@ -1460,7 +1458,7 @@ class PubDependencyTree {
   /// We then parse out the package name, version number, and sub-dependencies for
   /// each entry, and store than in our _versions and _dependencyTree fields
   /// above.
-  String fill(String message) {
+  String? fill(String message) {
     if (message.startsWith('- ')) {
       final int space2 = message.indexOf(' ', 2);
       int space3 = message.indexOf(' ', space2 + 1);
@@ -1497,19 +1495,20 @@ class PubDependencyTree {
   /// excluding any listed in `seen`.
   Iterable<String> getTransitiveDependenciesFor(
     String package, {
-    @required Set<String> seen,
-    @required Set<String> exclude,
-    List<String>/*?*/ result,
+    required Set<String> seen,
+    required Set<String> exclude,
+    List<String>? result,
   }) {
     assert(seen != null);
     assert(exclude != null);
     result ??= <String>[];
-    if (!_dependencyTree.containsKey(package)) {
+    final Set<String>? dependencies = _dependencyTree[package];
+    if (dependencies == null) {
       // We have no transitive dependencies extracted for flutter_sdk packages
       // because they were omitted from pubspec.yaml used for 'pub upgrade' run.
       return result;
     }
-    for (final String dependency in _dependencyTree[package]) {
+    for (final String dependency in dependencies) {
       if (!seen.contains(dependency)) {
         if (!exclude.contains(dependency)) {
           result.add(dependency);
@@ -1523,7 +1522,7 @@ class PubDependencyTree {
 
   /// The version that a particular package ended up with.
   String versionFor(String package) {
-    return _versions[package];
+    return _versions[package]!;
   }
 }
 
@@ -1535,7 +1534,6 @@ String _computeChecksum(Iterable<String> names, String Function(String name) get
   final List<String> sortedNames = names.toList()..sort();
   for (final String name in sortedNames) {
     final String version = getVersion(name);
-    assert(version != '');
     if (version == null) {
       continue;
     }
@@ -1589,7 +1587,7 @@ Directory createTemporaryFlutterSdk(
       .childDirectory(flutterPackage)
       .childFile('pubspec.yaml')
       ..createSync(recursive: true);
-    final PubspecYaml pubspecYaml = pubspecsByName[flutterPackage];
+    final PubspecYaml? pubspecYaml = pubspecsByName[flutterPackage];
     if (pubspecYaml == null) {
       logger.printWarning(
         "Unexpected package '$flutterPackage' found in packages directory",

packages/flutter_tools/lib/src/dart/pub.dart

@@ -31,7 +31,7 @@ const String _kPubCacheEnvironmentKey = 'PUB_CACHE';
 /// (see https://github.com/dart-lang/pub/blob/master/lib/src/exit_codes.dart)
 const int _kPubExitCodeUnavailable = 69;
 
-typedef MessageFilter = String Function(String message);
+typedef MessageFilter = String? Function(String message);
 
 /// Represents Flutter-specific data that is added to the `PUB_ENVIRONMENT`
 /// environment variable and allows understanding the type of requests made to
@@ -311,7 +311,7 @@ class _DefaultPub implements Pub {
 
     String lastPubMessage = 'no message';
     bool versionSolvingFailed = false;
-    String filterWrapper(String line) {
+    String? filterWrapper(String line) {
       lastPubMessage = line;
       if (line.contains('version solving failed')) {
         versionSolvingFailed = true;