Clarify that prefixes may not be built-in identifers

R=kustermann@google.com

Review URL: https://codereview.chromium.org//415333003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@38567 260f80e4-7a28-3924-810f-c04153c831b5
1 file changed