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

Some tests fail.

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

git-svn-id: 34d430745197549daf83b1f027ea5104cda4d1f7@35245 260f80e4-7a28-3924-810f-c04153c831b5

dart-archive/template-binding,dart-archive/template-binding,dart-archive/template-binding