Transition of platform channel test interfaces to flutter_test package

The flutter_test package provides some shims so that uses of the obsolete setMock... and checkMock... methods will continue to work. Tests that previously did not import package:flutter_test/flutter_test.dart can do so to enable these shims; this should be sufficient to migrate most code.

These shim APIs are deprecated, however. Instead, in code using WidgetTester (for example, using testWidgets), it is recommended to use the following patterns to replace calls to those methods (where tester is the WidgetTester instance):

// old code
ServicesBinding.defaultBinaryMessenger.setMockMessageHandler(...);
ServicesBinding.defaultBinaryMessenger.checkMockMessageHandler(...);
// new code
tester.binding.defaultBinaryMessenger.setMockMessageHandler(...);
tester.binding.defaultBinaryMessenger.checkMockMessageHandler(...);

// old code
myChannel.setMockMessageHandler(...);
myChannel.checkMockMessageHandler(...);
// new code
tester.binding.defaultBinaryMessenger.setMockDecodedMessageHandler(myChannel, ...);
tester.binding.defaultBinaryMessenger.checkMockMessageHandler(myChannel, ...);

// old code
myMethodChannel.setMockMethodCallHandler(...);
myMethodChannel.checkMockMethodCallHandler(...);
// new code
tester.binding.defaultBinaryMessenger.setMockMethodCallHandler(myMethodChannel, ...);
tester.binding.defaultBinaryMessenger.checkMockMessageHandler(myMethodChannel, ...);

Tests that use package:test and test() can be changed to use package:flutter_test and testWidgets() to get access to a WidgetTester.

Code that does not have access to a WidgetTester can refer to TestDefaultBinaryMessengerBinding.instance!.defaultBinaryMessenger instead of tester.binding.defaultBinaryMessenger.

Tests that do not use the default test widgets binding (AutomatedTestWidgetsFlutterBinding, which is initialized by testWidgets) can mix the TestDefaultBinaryMessengerBinding mixin into their binding to get the same results.

Tests that manipulate onPlatformMessage will no longer function as designed. To send mock messages to the framework, consider using ChannelBuffers.push. There is no mechanism to intercept messages from the plugins and forward them to the framework in the new API. If your use case requires such a mechanism, please file a bug.