Remove Expect from core library.

Committed: https://code.google.com/p/dart/source/detail?r=19755
Reverted: http://code.google.com/p/dart/source/detail?r=19756

Review URL: https://codereview.chromium.org//12212016

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/yaml@20996 260f80e4-7a28-3924-810f-c04153c831b5