Dart2js and the Dartvm seem inconsistent in the implementation of ClassMirror.metadata. Looking at the api, it seems that the vm behavior matches what's documented.

Here is a test case that can be used to repro this:

  import 'dart:mirrors';
  const meta = "annotation";

  @­meta
  class A {}

  class B extends A {}

  main() {
    print('A: ${reflectClass(A).metadata.length}');
    print('B: ${reflectClass(B).metadata.length}');
  }

The dartvm will print:
  A: 1
  B: 0

dart2js will print:
  A: 1
  B: 1