Discard the serialized AST before linking, to save memory.
Also, tweaks CompilationResult.compareToComparable() to
not read in the JavaScript files from disk, to speed things
up when memory is tight.

Review by: bobv



git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@4874 8db76d5a-ed1c-0410-87a9-c151d255dfc7