[NEW TYPE INFERENCE] Don't type vars in externs as undefined
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=77311079
[NEW TYPE INFERENCE] Don't treat an aliased final constructor as a const variable.

We still don't handle aliased constructors, but that's for a separate CL.
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=77357275