Avoid some false positives in the CheckNullableReturn lint check

Do not report false positives for methods:
- Overriding supertype methods with the same signature
- Containing a single "throw" (a means of abstract method implementation)

Fixes #656
They were pushed out here:
https://github.com/google/closure-compiler/commit/6968b5c9dbeb30ed36a5462c87bd280de99715c5