Add local overrides to testbed and provide more defaults (#33264)

packages/flutter_tools/test/src/testbed.dart

@@ -8,14 +8,22 @@ import 'package:file/memory.dart';
 import 'package:flutter_tools/src/base/context.dart';
 import 'package:flutter_tools/src/base/file_system.dart';
 import 'package:flutter_tools/src/base/logger.dart';
+import 'package:flutter_tools/src/base/terminal.dart';
 import 'package:flutter_tools/src/cache.dart';
 import 'package:flutter_tools/src/context_runner.dart';
 
+import 'context.dart';
+
 export 'package:flutter_tools/src/base/context.dart' show Generator;
 
+// A default value should be provided if one of the following criteria is met:
+//    - The vast majority of tests should use this provider. For example,
+//      [BufferLogger], [MemoryFileSystem].
+//    - More TBD.
 final Map<Type, Generator> _testbedDefaults = <Type, Generator>{
-  FileSystem: () => MemoryFileSystem(),
-  Logger: () => BufferLogger(),
+  FileSystem: () => MemoryFileSystem(), // Keeps tests fast by avoid actual file system.
+  Logger: () => BufferLogger(), // Allows reading logs and prevents stdout.
+  OutputPreferences: () => OutputPreferences(showColor: false), // configures BufferLogger to avoid color codes.
 };
 
 /// Manages interaction with the tool injection and runner system.
@@ -61,11 +69,19 @@ class Testbed {
   final Map<Type, Generator> _overrides;
 
   /// Runs `test` within a tool zone.
-  FutureOr<T> run<T>(FutureOr<T> Function() test) {
+  ///
+  /// `overrides` may be used to provide new context values for the single test
+  /// case or override any context values from the setup.
+  FutureOr<T> run<T>(FutureOr<T> Function() test, {Map<Type, Generator> overrides}) {
     final Map<Type, Generator> testOverrides = Map<Type, Generator>.from(_testbedDefaults);
+    // Add the initial setUp overrides
     if (_overrides != null) {
       testOverrides.addAll(_overrides);
     }
+    // Add the test-specific overrides
+    if (overrides != null) {
+      testOverrides.addAll(overrides);
+    }
     // Cache the original flutter root to restore after the test case.
     final String originalFlutterRoot = Cache.flutterRoot;
     return runInContext<T>(() {

packages/flutter_tools/test/testbed_test.dart

+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:file/memory.dart';
+import 'package:flutter_tools/src/base/context.dart';
+import 'package:flutter_tools/src/base/file_system.dart';
+
+import 'src/common.dart';
+import 'src/testbed.dart';
+
+void main() {
+  group('Testbed', () {
+
+    test('Can provide default interfaces', () async {
+      final Testbed testbed = Testbed();
+
+      FileSystem localFileSystem;
+      await testbed.run(() {
+        localFileSystem = fs;
+      });
+
+      expect(localFileSystem, isA<MemoryFileSystem>());
+    });
+
+    test('Can provide setup interfaces', () async {
+      final Testbed testbed = Testbed(overrides: <Type, Generator>{
+        A: () => A(),
+      });
+
+      A instance;
+      await testbed.run(() {
+        instance = context.get<A>();
+      });
+
+      expect(instance, isA<A>());
+    });
+
+    test('Can provide local overrides', () async {
+      final Testbed testbed = Testbed(overrides: <Type, Generator>{
+        A: () => A(),
+      });
+
+      A instance;
+      await testbed.run(() {
+        instance = context.get<A>();
+      }, overrides: <Type, Generator>{
+        A: () => B(),
+      });
+
+      expect(instance, isA<B>());
+    });
+  });
+}
+
+class A {}
+
+class B extends A {}