Revert "Remove unmodifiable map wrappers that could instead use UnmodifiableMapView."

Some tests fail.

Review URL: https://codereview.chromium.org//246833003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/yaml@35245 260f80e4-7a28-3924-810f-c04153c831b5
1 file changed
tree: 86f6b50bc21b5cfd3bc7b8e386a05b2db6ba7792
  1. pkgs/