This patch modifies the compiler in two ways:
1. It can shard out precompiles during parallel builds.
2. Linkers can indicate which part should happen per-shard and which part should
happen on the final link node.

http://gwt-code-reviews.appspot.com/141811

Review by: cromwellian


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@7650 8db76d5a-ed1c-0410-87a9-c151d255dfc7