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

runtime/vm/code_generator_test.cc

Index: runtime/vm/code_generator_test.cc
===================================================================
--- runtime/vm/code_generator_test.cc	(revision 25114)
+++ runtime/vm/code_generator_test.cc	(working copy)
@@ -272,7 +272,7 @@
 
 static RawClass* LookupClass(const Library& lib, const char* name) {
   const String& cls_name = String::ZoneHandle(Symbols::New(name));
-  return lib.LookupClass(cls_name);
+  return lib.LookupClass(cls_name, NULL);
 }
 
 
@@ -552,7 +552,7 @@
   app_lib ^= libs.At(num_libs - 1);
   ASSERT(!app_lib.IsNull());
   const Class& cls = Class::Handle(
-      app_lib.LookupClass(String::Handle(Symbols::New("A"))));
+      app_lib.LookupClass(String::Handle(Symbols::New("A")), NULL));
   EXPECT_EQ(cls.raw(), result.clazz());
 }