Update docs on @observable to hint about mixin Observable.

R=jmesserly@google.com

Review URL: https://codereview.chromium.org//368793002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/observe@37955 260f80e4-7a28-3924-810f-c04153c831b5
2 files changed
tree: 3579686eaaa2814118dbfcca9bd42b608f1fe246
  1. lib/
  2. test/
  3. AUTHORS
  4. CHANGELOG.md
  5. LICENSE
  6. PATENTS
  7. pubspec.yaml