Only parse the directives when rewriting imports.

This shaves about 13% (a little under a second) off the time it
takes to load the asset environment for TodoMVC in my tests.

R=brianwilkerson@google.com, nweiz@google.com

Review URL: https://codereview.chromium.org//314583007

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@36960 260f80e4-7a28-3924-810f-c04153c831b5