Ensure Flutter for Web debug builds are only accessible through Flutter run (#40783)

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

@@ -17,7 +17,7 @@ class BuildWebCommand extends BuildSubCommand {
   BuildWebCommand() {
     usesTargetOption();
     usesPubOption();
-    addBuildModeFlags();
+    addBuildModeFlags(excludeDebug: true);
     argParser.addFlag('web-initialize-platform',
         defaultsTo: true,
         negatable: true,
@@ -50,6 +50,9 @@ class BuildWebCommand extends BuildSubCommand {
     final FlutterProject flutterProject = FlutterProject.current();
     final String target = argResults['target'];
     final BuildInfo buildInfo = getBuildInfo();
+    if (buildInfo.isDebug) {
+      throwToolExit('debug builds cannot be built directly for the web. Try using "flutter run"');
+    }
     await buildWeb(flutterProject, target, buildInfo, argResults['web-initialize-platform']);
     return null;
   }

packages/flutter_tools/lib/src/runner/flutter_command.dart

@@ -126,6 +126,8 @@ abstract class FlutterCommand extends Command<void> {
 
   bool get shouldUpdateCache => true;
 
+  bool _excludeDebug = false;
+
   BuildMode _defaultBuildMode;
 
   void requiresPubspecYaml() {
@@ -263,12 +265,17 @@ abstract class FlutterCommand extends Command<void> {
             'Normally there\'s only one, but when adding Flutter to a pre-existing app it\'s possible to create multiple.');
   }
 
-  void addBuildModeFlags({ bool defaultToRelease = true, bool verboseHelp = false }) {
+  void addBuildModeFlags({ bool defaultToRelease = true, bool verboseHelp = false, bool excludeDebug = false }) {
+    // A release build must be the default if a debug build is not possible.
+    assert(defaultToRelease || !excludeDebug);
+    _excludeDebug = excludeDebug;
     defaultBuildMode = defaultToRelease ? BuildMode.release : BuildMode.debug;
 
-    argParser.addFlag('debug',
-      negatable: false,
-      help: 'Build a debug version of your app${defaultToRelease ? '' : ' (default mode)'}.');
+    if (!excludeDebug) {
+      argParser.addFlag('debug',
+        negatable: false,
+        help: 'Build a debug version of your app${defaultToRelease ? '' : ' (default mode)'}.');
+    }
     argParser.addFlag('profile',
       negatable: false,
       help: 'Build a version of your app specialized for performance profiling.');
@@ -312,11 +319,12 @@ abstract class FlutterCommand extends Command<void> {
   }
 
   BuildMode getBuildMode() {
-    final List<bool> modeFlags = <bool>[argResults['debug'], argResults['profile'], argResults['release']];
+    final bool debugResult = _excludeDebug ? false : argResults['debug'];
+    final List<bool> modeFlags = <bool>[debugResult, argResults['profile'], argResults['release']];
     if (modeFlags.where((bool flag) => flag).length > 1) {
       throw UsageException('Only one of --debug, --profile, or --release can be specified.', null);
     }
-    if (argResults['debug']) {
+    if (debugResult) {
       return BuildMode.debug;
     }
     if (argResults['profile']) {

packages/flutter_tools/test/general.shard/commands/build_web_test.dart

@@ -92,6 +92,15 @@ void main() {
     );
   }));
 
+  test('Refuses to build a debug build for web', () => testbed.run(() async {
+    final CommandRunner<void> runner = createTestCommandRunner(BuildCommand());
+
+    expect(() => runner.run(<String>['build', 'web', '--debug']),
+        throwsA(isInstanceOf<UsageException>()));
+  }, overrides: <Type, Generator>{
+    FeatureFlags: () => TestFeatureFlags(isWebEnabled: true),
+  }));
+
   test('Refuses to build for web when feature is disabled', () => testbed.run(() async {
     final CommandRunner<void> runner = createTestCommandRunner(BuildCommand());