This posed a problem for the implementation of the legacy
testing APIs which, for historical reasons,
were previously in the flutter package.
Since they relied on the underlying logic being partly synchronous,
they required refactoring.
To avoid adding even more test logic into the flutter package,
a decision was made to move this logic to the flutter_test package.

Description of change

Specifically, the following APIs were affected:

BinaryMessenger.checkMessageHandler: Obsolete.

BinaryMessenger.setMockMessageHandler: Replaced by
TestDefaultBinaryMessenger.setMockMessageHandler.

BinaryMessenger.checkMockMessageHandler: Replaced
 by TestDefaultBinaryMessenger.checkMockMessageHandler.

BasicMessageChannel.setMockMessageHandler: Replaced
 by TestDefaultBinaryMessenger.setMockDecodedMessageHandler.

MethodChannel.checkMethodCallHandler: Obsolete.