Fix issue with --web-renderer. (#69319)

packages/flutter_tools/lib/src/isolated/devfs_web.dart

@@ -814,7 +814,7 @@ class WebDevFS implements DevFS {
       webAssetServer.webRenderer = WebRendererMode.autoDetect;
     } else if (buildInfo.dartDefines.contains('FLUTTER_WEB_USE_SKIA=true')) {
         webAssetServer.webRenderer = WebRendererMode.canvaskit;
-      }
+    }
     if (hostname == 'any') {
       _baseUri = Uri.http('localhost:$port', '');
     } else {

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

@@ -838,12 +838,12 @@ abstract class FlutterCommand extends Command<void> {
       ? stringArg(FlutterOptions.kPerformanceMeasurementFile)
       : null;
 
-    final List<String> dartDefines = argParser.options.containsKey(FlutterOptions.kDartDefinesOption)
+    List<String> dartDefines = argParser.options.containsKey(FlutterOptions.kDartDefinesOption)
         ? stringsArg(FlutterOptions.kDartDefinesOption)
         : <String>[];
 
     if (argParser.options.containsKey('web-renderer') && argResults.wasParsed('web-renderer')) {
-      _updateDartDefines(dartDefines, stringArg('web-renderer'));
+      dartDefines = updateDartDefines(dartDefines, stringArg('web-renderer'));
     }
 
     return BuildInfo(buildMode,
@@ -944,12 +944,15 @@ abstract class FlutterCommand extends Command<void> {
   }
 
   /// Updates dart-defines based on [webRenderer].
-  void _updateDartDefines(List<String> dartDefines, String webRenderer) {
-    if (dartDefines.any((String d) => d.startsWith('FLUTTER_WEB_USE_SKIA='))) {
-      throwToolExit('Only one of "--web-renderer" and '
-          '"--dart-defines=FLUTTER_WEB_USE_SKIA" may be specified.');
+  @visibleForTesting
+  static List<String> updateDartDefines(List<String> dartDefines, String webRenderer) {
+    final Set<String> dartDefinesSet = dartDefines.toSet();
+    if (!dartDefines.any((String d) => d.startsWith('FLUTTER_WEB_AUTO_DETECT='))
+        && dartDefines.any((String d) => d.startsWith('FLUTTER_WEB_USE_SKIA='))) {
+      dartDefinesSet.removeWhere((String d) => d.startsWith('FLUTTER_WEB_USE_SKIA='));
     }
-    dartDefines.addAll(_webRendererDartDefines[webRenderer]);
+    dartDefinesSet.addAll(_webRendererDartDefines[webRenderer]);
+    return dartDefinesSet.toList();
   }
 
   void _registerSignalHandlers(String commandPath, DateTime startTime) {

packages/flutter_tools/test/commands.shard/hermetic/run_test.dart

@@ -505,6 +505,47 @@ void main() {
       DeviceManager: () => mockDeviceManager,
     });
   });
+
+  group('dart-defines and web-renderer options', () {
+    List<String> dartDefines;
+
+    setUp(() {
+      dartDefines = <String>[];
+    });
+
+    test('auto web-renderer with no dart-defines', () {
+      dartDefines = FlutterCommand.updateDartDefines(dartDefines, 'auto');
+      expect(dartDefines, <String>['FLUTTER_WEB_AUTO_DETECT=true']);
+    });
+
+    test('canvaskit web-renderer with no dart-defines', () {
+      dartDefines = FlutterCommand.updateDartDefines(dartDefines, 'canvaskit');
+      expect(dartDefines, <String>['FLUTTER_WEB_AUTO_DETECT=false','FLUTTER_WEB_USE_SKIA=true']);
+    });
+
+    test('html web-renderer with no dart-defines', () {
+      dartDefines = FlutterCommand.updateDartDefines(dartDefines, 'html');
+      expect(dartDefines, <String>['FLUTTER_WEB_AUTO_DETECT=false','FLUTTER_WEB_USE_SKIA=false']);
+    });
+
+    test('auto web-renderer with existing dart-defines', () {
+      dartDefines = <String>['FLUTTER_WEB_USE_SKIA=false'];
+      dartDefines = FlutterCommand.updateDartDefines(dartDefines, 'auto');
+      expect(dartDefines, <String>['FLUTTER_WEB_AUTO_DETECT=true']);
+    });
+
+    test('canvaskit web-renderer with no dart-defines', () {
+      dartDefines = <String>['FLUTTER_WEB_USE_SKIA=false'];
+      dartDefines = FlutterCommand.updateDartDefines(dartDefines, 'canvaskit');
+      expect(dartDefines, <String>['FLUTTER_WEB_AUTO_DETECT=false','FLUTTER_WEB_USE_SKIA=true']);
+    });
+
+    test('html web-renderer with no dart-defines', () {
+      dartDefines = <String>['FLUTTER_WEB_USE_SKIA=true'];
+      dartDefines = FlutterCommand.updateDartDefines(dartDefines, 'html');
+      expect(dartDefines, <String>['FLUTTER_WEB_AUTO_DETECT=false','FLUTTER_WEB_USE_SKIA=false']);
+    });
+  });
 }
 
 class MockCache extends Mock implements Cache {}

packages/flutter_tools/test/general.shard/web/devfs_web_test.dart

@@ -942,12 +942,12 @@ void main() {
       useSseForDebugBackend: true,
       nullAssertions: true,
       buildInfo: const BuildInfo(
-          BuildMode.debug,
-          '',
-          treeShakeIcons: false,
-          dartDefines: <String>[
-            'FLUTTER_WEB_AUTO_DETECT=true',
-          ]
+        BuildMode.debug,
+        '',
+        treeShakeIcons: false,
+        dartDefines: <String>[
+          'FLUTTER_WEB_AUTO_DETECT=true',
+        ]
       ),
       enableDwds: false,
       entrypoint: Uri.base,