Remove trailing whitespace
Add a transpilation pass that runs before any other passes, to convert an AST containing ES6 features into one that only contains ES3 features, if the language_in is ES6 and the language_out is ES3 or ES5.

Once all the existing passes support ES6 features, we may move this new pass to the end of the compilation, just before code printing, but for now we do it at the beginning so the existing passes will still work.

So far, this just converts extended object literals ({a, b}) to ES3 object literals ({a: a, b: b}).

See issue #406.
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=66553755