[Extension type] VM debugging support

[itsjustkevin]

[eernstg]

Please note that this feature has been renamed: It is now the 'inline class' feature.

[Edit Sep 2023: It has been renamed again, it is now 'extension types'.]

[a-siva]

Looks like some work on expression evaluation is required.
Consider this example

extension type V0(int id) {
  num foo() => 42;
}
extension type V1(int id) implements V0 {}
extension type V2(int id) implements V0 {}
extension type V(int id) implements V1, V2 {}
main() {
  V v = V(1);
  V1 v1 = V1(1);
  V2 v2 = V2(1);
  print(v.foo());
  print(v1.foo());
  print(v2.foo());
}

Inside devtools if we evaluate
v.foo()
we get the following error

evaluateInFrame: (113) Expression compilation error org-dartlang-debug:synthetic_debug_expression:1:3:
Error: The method 'foo' isn't defined for the class '_Smi'. 
 '_Smi' is from 'dart:core'.

[a-siva]

When we hover the cursor on V/V1/V2 it shows the type as int, maybe it needs to indicate it is an extension type wrapping int``

[a-siva]

//cc @derekxu16 @bkonyi