Confusing error message from analyzer on incomplete mixinApplication

[eernstg]

Cf. #36946, consider the following program:

class A<X> {}
class test<X extends num> = A<X>;
main() {}

With the analyzer (from e13b1b6) we get the following response:

ERROR|COMPILE_TIME_ERROR|MIXIN_OF_NON_CLASS|/usr/local/google/home/eernst/lang/dart/scratch/201905/n007.dart|2|33|0|Classes can only mixin other classes.
ERROR|SYNTACTIC_ERROR|EXPECTED_TOKEN|/usr/local/google/home/eernst/lang/dart/scratch/201905/n007.dart|2|33|1|Expected to find 'with'.
ERROR|STATIC_WARNING|UNDEFINED_CLASS|/usr/local/google/home/eernst/lang/dart/scratch/201905/n007.dart|2|33|0|Undefined class ''.

It looks like the analyzer sees A<X> and expects with (because it's parsing a <mixinApplication>); then it repairs the program by inventing with N where N is the empty identifier, and then it complains that N is not a class. It might be more helpful for developers to just report a syntax error in the <mixinApplication>, or at least to avoid describing properties of syntax that the developer did not write.

[srawlins]

The "undefined class" error is gone. The analyzer now reports:

• error • Classes can only mix in mixins and classes. • a.dart:2:33 • mixin_of_non_class
• error • Expected to find 'with'. • a.dart:2:33 • expected_token

so still not ideal.