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

R=sgjesse@google.com

Review URL: https://codereview.chromium.org//214723002

git-svn-id: d894903a6691b4329a8535ed724b8f115331b4ef@35244 260f80e4-7a28-3924-810f-c04153c831b5

dart-lang/yaml