Allowing adding/updating packages during hot reload (#29747)

packages/flutter_tools/lib/src/devfs.dart

@@ -481,6 +481,9 @@ class DevFS {
       outputPath:  dillOutputPath ?? getDefaultApplicationKernelPath(trackWidgetCreation: trackWidgetCreation),
       packagesFilePath : _packagesFilePath,
     );
+    if (compilerOutput == null) {
+      return UpdateFSReport(success: false);
+    }
     // list of sources that needs to be monitored are in [compilerOutput.sources]
     sources = compilerOutput.sources;
     //

packages/flutter_tools/lib/src/run_hot.dart

@@ -317,7 +317,9 @@ class HotRunner extends ResidentRunner {
     // for all devices should be in sync.
     final List<Uri> invalidatedFiles = ProjectFileInvalidator.findInvalidated(
       lastCompiled: flutterDevices[0].devFS.lastCompiled,
-      urisToMonitor: flutterDevices[0].devFS.sources);
+      urisToMonitor: flutterDevices[0].devFS.sources,
+      packagesPath: packagesFilePath,
+    );
     final UpdateFSReport results = UpdateFSReport(success: true);
     for (FlutterDevice device in flutterDevices) {
       results.incorporateResults(await device.updateDevFS(
@@ -939,8 +941,11 @@ class ProjectFileInvalidator {
   static const String _pubCachePathLinuxAndMac = '.pub-cache';
   static const String _pubCachePathWindows = 'Pub/Cache';
 
-  static List<Uri> findInvalidated({@required DateTime lastCompiled,
-    @required List<Uri> urisToMonitor}) {
+  static List<Uri> findInvalidated({
+    @required DateTime lastCompiled,
+    @required List<Uri> urisToMonitor,
+    @required String packagesPath,
+  }) {
     final List<Uri> invalidatedFiles = <Uri>[];
     int scanned = 0;
     final Stopwatch stopwatch = Stopwatch()..start();
@@ -960,6 +965,13 @@ class ProjectFileInvalidator {
         invalidatedFiles.add(uri);
       }
     }
+    // we need to check the .packages file too since it is not used in compilation.
+    final DateTime packagesUpdatedAt = fs.statSync(packagesPath).modified;
+    if (lastCompiled != null && packagesUpdatedAt != null
+        && packagesUpdatedAt.millisecondsSinceEpoch > lastCompiled.millisecondsSinceEpoch) {
+      invalidatedFiles.add(fs.file(packagesPath).uri);
+      scanned++;
+    }
     printTrace('Scanned through $scanned files in ${stopwatch.elapsedMilliseconds}ms');
     return invalidatedFiles;
   }

packages/flutter_tools/test/project_file_invalidator_test.dart

@@ -14,7 +14,7 @@ void main() {
     final MemoryFileSystem memoryFileSystem = MemoryFileSystem();
     testUsingContext('Empty project', () async {
       expect(
-        ProjectFileInvalidator.findInvalidated(lastCompiled: DateTime.now(), urisToMonitor: <Uri>[]),
+        ProjectFileInvalidator.findInvalidated(lastCompiled: DateTime.now(), urisToMonitor: <Uri>[], packagesPath: ''),
         isEmpty);
     }, overrides: <Type, Generator>{
       FileSystem: () => memoryFileSystem,
@@ -24,7 +24,9 @@ void main() {
       expect(
         ProjectFileInvalidator.findInvalidated(
             lastCompiled: DateTime.now(),
-            urisToMonitor: <Uri>[Uri.parse('/not-there-anymore')]),
+            urisToMonitor: <Uri>[Uri.parse('/not-there-anymore'),],
+            packagesPath: '',
+          ),
         isEmpty);
     }, overrides: <Type, Generator>{
       FileSystem: () => memoryFileSystem,