Add option to get only reference.

This allows quick lookups for addresses of very long lists, for instance.

R=johnmccutchan@google.com

Review URL: https://codereview.chromium.org//812123002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42444 260f80e4-7a28-3924-810f-c04153c831b5
Force first DartToolsPlugin.getImageDescriptorRegistry() invocation in the UI thread.

Otherwise it may try to initialize in the code completion handler, i.e. in background thread.

R=danrubel@google.com
BUG=

Review URL: https://codereview.chromium.org//807173006

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42445 260f80e4-7a28-3924-810f-c04153c831b5