Fix dynamic_prefix_core_test to reflect that dynamic is a member of dart:core

[eernstg]

This issue is concerned with the test tests/language_2/dynamic_prefix_core_test.dart: It should be updated such that it reflects the fact that dynamic is a member of 'dart:core'. It could be modified to be similar to the following:

// With an explicit import of 'dart:core', test that `dynamic` is
// treated as a member of that library.

import "package:expect/expect.dart";
import "dart:core" as mycore;

void main() {
  // `dynamic` is undefined.
  dynamic x; //# 01: compile-time error
  dynamic; //# 02: compile-time error

  // `mycore.dynamic` is the type we normally know as `dynamic`
  Expect.isTrue(mycore.dynamic is mycore.Type); //# 03: ok
  Expect.isTrue(<mycore.Object>[] is mycore.List<mycore.dynamic>); //# 04: ok
}

If we have policies against making any changes to tests/language at all then only the one in language_2 needs to be updated, otherwise tests/language/dynamic_prefix_core_test.dart should be modified similarly (if Dart 1 tests are being updated at all now).

Status entries should not refer to issue 12478 which says that dynamic is not exported by 'dart:core' and the test ought to work as-is (but the vm won't be updated accordingly because it is expected that the common front end will contain the update). I added a comment on 12478 saying that it is the test that needs to be updated, pointing to this issue.

Cf. #29125 for more details about the background.

[rmacnak-google]

The mirrors implementations will also need to be updated to reflect this change.