add symbol support in callsTo to facilitate rename refactoring

This PR allows you to pass in symbol literals for method names instead of strings in the hope that that will make the mock tests more rename refactoring friendly. 
See 
https://code.google.com/p/dart/issues/detail?id=15793&can=4&colspec=ID%20Type%20Status%20Priority%20Area%20Milestone%20Owner%20Summary

Note mock_test.dart had one method marked as solo_test. I changed that first and discovered you had a breaking test ("Mocking: instances")

cheers
Anders