Turn Iterators.emptyModifiableIterator into an enum singleton instead of an anonymous inner class.
Turn Iterators.emptyModifiableIterator into an enum singleton instead of an anonymous inner class.

ProGuard can't eliminate static fields, but it can recognize that the enum isn't referenced except by emptyModifiableIterator and eliminate it from classes that reference other methods in Iterators.

Effects on ImmutableMapProGuard: ([]41126 bytes => 40551 bytes
40 classes => 39 classes
247 methods => 243 methods

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=146953456

EdwardLee03/guava,google/guava,typetools/guava,berndhopp/guava,typetools/guava,DavesMan/guava,xasx/guava,rgoldberg/guava,EdwardLee03/guava,xasx/guava,Xaerxess/guava,mosoft521/guava,mosoft521/guava,google/guava,rgoldberg/guava,DavesMan/guava,mosoft521/guava,Xaerxess/guava,berndhopp/guava,google/guava