Improve import prefix search results UI.

1. Bug fix for search on an import directive without a prefix.

2. Pretend that import prefixes are children on libraries' compilation units.

3. Icon for import prefix elements in Editor.

R=brianwilkerson@google.com
BUG=

Review URL: https://codereview.chromium.org//640333002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41024 260f80e4-7a28-3924-810f-c04153c831b5