Type check constant variable declarations.

BUG=dartbug.com/16391
R=brianwilkerson@google.com

Review URL: https://codereview.chromium.org//637303005

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41054 260f80e4-7a28-3924-810f-c04153c831b5