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

In addition, the onPlatformMessage callback,
which previously was hooked by the framework to
receive messages from plugins, is no longer used
(and will be removed in due course). As a result,
calling this callback to inject messages into the
framework no longer has an effect.

Migration guide

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