@export on a Polymer method just adds the method to "Object.prototype"

I noticed this while fixing a bug in GenerateExports: If you add `@export` to a method of a Polymer element that is defined in the Polymer 1 style (`Polymer({ method1() {} method2() {} })`) the compiler just adds that method to `Object.prototype` in the externs, rather than generating a `goog.exportSymbol` call. This means that other properties with the same name won't get renamed, so code size might suffer. If a Polymer project finds their code size is too big this is one place we might look. Until then, I'm guessing this is a relatively low-priority issue.

cc @rictic