MethodChannel.checkMethodCallHandler: Obsolete.

MethodChannel.setMockMethodCallHandler: Replaced by
 TestDefaultBinaryMessenger.setMockMethodCallHandler.

MethodChannel.checkMockMethodCallHandler: Replaced
 by TestDefaultBinaryMessenger.checkMockMessageHandler.

These replacements are only available to code using the
new TestDefaultBinaryMessengerBinding
(such as any code using testWidgets in a flutter_test test).
There is no replacement for production code that was using
these APIs, as they were not intended for production code use.

Tests using checkMessageHandler have no equivalent in the
new API, since message handler registration is handled
directly by the ChannelBuffers object, which does not
expose the currently registered listener for a channel.
(Tests verifying handler registration appear to be rare.)

Code that needs migrating may see errors such as the following: