Tweak text to clarify that using atype variable in an expression in a static method is a compile time error.

R=regis@google.com

Review URL: https://codereview.chromium.org//136473013

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@31940 260f80e4-7a28-3924-810f-c04153c831b5