Analyzer: Remove error for library name clash

[eernstg]

The analyzer reports an error for the following situation:

// Library 'lib1.dart'.
library clashing.nonempty.name;

// Library 'lib2.dart'.
library clashing.nonempty.name;

// Library 'main.dart'.
import 'lib1.dart';
import 'lib2.dart';

void main() {}

This was specified as a warning, but was removed from the language specification in dart-lang/language#1083, so the analyzer should no more report this error, with or without null-safety.

[eernstg]

@johnniwinther found an extra error which is very similar. Cf. dart-lang/language#1096.

[eernstg]

dart-lang/language#1096 has now been landed, so the error for library name clashes in exports should also be eliminated.

[scheglov]

https://dart-review.googlesource.com/c/sdk/+/154540

[scheglov]

I opened a new co19 issue dart-lang/co19#837