Remove unmodifiable map wrappers that could instead use UnmodifiableMapView. R=sgjesse@google.com Committed: https://code.google.com/p/dart/source/detail?r=35244 Review URL: https://codereview.chromium.org//214723002 git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/yaml@35361 260f80e4-7a28-3924-810f-c04153c831b5