Analyzer: Wrong error message about `async` return type

[eernstg]

Consider the following library:

Future<bool> f() async => Future<int>.value(42);

dartanalyzer --enable-experiment=non-nullable from commit d5310cb (of today) responds with the following error message:

ERROR|STATIC_TYPE_WARNING|RETURN_OF_INVALID_TYPE|/usr/local/google/home/eernst/lang/dart/scratch/202004/n039.dart|1|27|21|A value of type 'int' can't be returned from function 'f' because it has a return type of 'bool'.

This error message indicates that the return type of f is bool, but it is actually declared as Future<bool>, so that error message needs to be adjusted. The same behavior exists also without --enable-experiment=non-nullable.

Discussions about async returns are ongoing, cf. dart-lang/language#941. However, I do not think that the outcome of that PR will invalidate a fix for this issue.

[scheglov]

With 0861470 analyzer now reports:
ERROR|STATIC_TYPE_WARNING|RETURN_OF_INVALID_TYPE|/Users/scheglov/dart/test/bin/test.dart|1|27|21|A value of type 'Future<int>' can't be returned from function 'f' because it has a return type of 'Future<bool>'.