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

runtime/vm/code_descriptors_test.cc

Index: runtime/vm/code_descriptors_test.cc
===================================================================
--- runtime/vm/code_descriptors_test.cc	(revision 25321)
+++ runtime/vm/code_descriptors_test.cc	(working copy)
@@ -225,9 +225,10 @@
   const String& name = String::Handle(String::New(TestCase::url()));
   const Library& lib = Library::Handle(Library::LookupLibrary(name));
   EXPECT(!lib.IsNull());
+  String& ambiguity_error_msg = String::Handle();
   Class& cls = Class::Handle(
-      lib.LookupClass(String::Handle(Symbols::New("A"))));
-  EXPECT(!cls.IsNull());
+      lib.LookupClass(String::Handle(Symbols::New("A")), &ambiguity_error_msg));
+  EXPECT(!cls.IsNull());  // No ambiguity error expected.
 
   // Now compile the two functions 'A.foo' and 'A.moo'
   String& function_moo_name = String::Handle(String::New("moo"));