Refactor resolution code in the vm to properly handle ambiguity errors.

runtime/vm/snapshot.cc

Index: runtime/vm/snapshot.cc
===================================================================
--- runtime/vm/snapshot.cc	(revision 25321)
+++ runtime/vm/snapshot.cc	(working copy)
@@ -212,7 +212,7 @@
   library_ = Library::LookupLibrary(str_);
   ASSERT(!library_.IsNull());
   str_ ^= ReadObjectImpl();
-  cls = library_.LookupClass(str_);
+  cls = library_.LookupClass(str_, NULL);  // No ambiguity error expected.
   cls.EnsureIsFinalized(isolate());
   ASSERT(!cls.IsNull());
   return cls.raw();