Protobuf changes for smaller dart2js code, Int64 fixes

This change is paired with https://chromiumcodereview.appspot.com/814213003

Reduces code size for one app by 0.9%

1. Allow constants for the default value to avoid many trivial closures.
2. Generate and use static M.create() and M.createRepeated() methods on message classes M to ensure there is a shared copy of these closures rather than one copy per use.
3. Parse Int64 values rather than generate from 'int' to ensure no truncation errors in JavaScript.

R=sigmund@google.com

Review URL: https://chromiumcodereview.appspot.com//813373003