consider renaming deduplicated mixins and placing them into artificial libraries

[mraleph]

Consider the following code:

// In package:something/one.dart
library one; 

class OneClass extends B with C { }

// In package:something_else/other.dart
library other;

class OtherClass extends B with C { }

// In package:main/main.dart
library main;

main() {
  B();
}

Due to mixing deduplication it is possible that OtherClass would be rewritten to extend _OneClass&B&C in library one - and in the resulting binary there would be objects representing Library package:something/one.dart - even though this library is not actually used.

This might be very confusing and hard to understand for users. We should consider renaming _OneClass&B&C to make class name decoupled from OneClass and we should consider moving it into an artificial library (e.g. dart:deduplicated_mixins or just dart:_internal).

Internal bug that prompted investigation: b/165124315

[mraleph]

/cc @alexmarkov

[mraleph]

Alternatively we could just move class at the end of the compilation - when we know which ones of the application survived second round of tree shaking at that point if it's the last thing in the library we can just take it and move it to another library.

[lfkdsk]

@mraleph #43172 try to do this task.