Implemented yield to return undefined if no expression follows it in a generator.
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=71467208
Add FontFaceSet#has to the javascript externs and update some existing documentation.
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=71571356
[NEW TYPE INFERENCE] Cache and reuse some types during GlobalTypeInfo to save mem.
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=71587749
Add support for yield as an expression in generators.
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=71588702
Fix bug in MinimizeExitPoints which causes us to remove a 'break' resulting in incorrect code.

Fixes #554 on Github.
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=71589594
Added a check to disallow labels inside generators.
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=71594344