Fail the compile if a linker lacks a @LinkerOrder annotation.

(This was an assert before, but asserts don't help developers
debug linker issues.)

Change-Id: Ib22fd02bceaaa3621e281cfe9476844284b3a413
Listens for file changes to shortcut file resources rescans.

Change-Id: I558c2f4526e0ca02e6e84c8a5c2b4c33b100af1b
Review-Link: https://gwt-review.googlesource.com/#/c/10160/
Deletes some dead code.

During library compilation development it was expected that 
CompilationUnit retrieval would be proxied through a 
LibraryGroupUnitCache, but it turned out not to be useful.

Change-Id: I56691fe763a05a750f1112fb854f9b83cf2c6299