fix tests on Windows for flutter tools (#8322)

packages/flutter_tools/test/src/context.dart

@@ -42,7 +42,7 @@ void testUsingContext(String description, dynamic testMethod(), {
 
     // Initialize the test context with some default mocks.
     // Seed these context entries first since others depend on them
-    testContext.putIfAbsent(Platform, () => new _FakePlatform());
+    testContext.putIfAbsent(Platform, () => const LocalPlatform());
     testContext.putIfAbsent(FileSystem, () => const LocalFileSystem());
     testContext.putIfAbsent(ProcessManager, () => const LocalProcessManager());
     testContext.putIfAbsent(Logger, () => new BufferLogger());
@@ -140,13 +140,6 @@ class MockSimControl extends Mock implements SimControl {
   }
 }
 
-class _FakePlatform extends FakePlatform {
-  _FakePlatform() : super.fromPlatform(const LocalPlatform());
-
-  @override
-  bool get isWindows => false;
-}
-
 class MockOperatingSystemUtils extends Mock implements OperatingSystemUtils {}
 
 class MockIOSSimulatorUtils extends Mock implements IOSSimulatorUtils {}