`Pointer.fromFunction`'s exceptional return const detection doesn't work very well

[liamappelbe]

A user reported some bugs with Pointer.fromFunction's exceptional return param: dart-lang/native#1724

There are a few different bugs here, but they're all to do with the const detection. The following all erroneously produce a "Exceptional return value must be a constant" error when passed to exceptional return:

• Negative int literals. These should just work.
• Simple enums values. These should also just work.
• An enhanced enum. This should produce a type mismatch error, but is reported as the same constness error, so the user didn't know that they were supposed to pass dc_status_t.DC_STATUS_TIMEOUT.value.
• The enhanced enum's .value field doesn't work anyway.

[dart-github-bot]

Summary: Pointer.fromFunction incorrectly flags negative integers, enums, and enhanced enum values as non-constant when used with exceptional return parameters. Const detection needs improvement.

[dcharkes]

Duplicate of:

• argument_must_be_a_constant for literal -1 passed to exceptionalReturn in NativeCallable.isolateLocal #54417