Remove the option to turn off the inferConst pass.
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=69089088
Add support for basic rewrite of lets and consts.

Loop closures are not supported yet. Classes are not handled yet. Functions might not be handled correctly yet.
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=69129845