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

runtime/vm/exceptions.cc

Index: runtime/vm/exceptions.cc
===================================================================
--- runtime/vm/exceptions.cc	(revision 25291)
+++ runtime/vm/exceptions.cc	(working copy)
@@ -394,7 +394,8 @@
 RawInstance* Exceptions::NewInstance(const char* class_name) {
   const String& cls_name = String::Handle(Symbols::New(class_name));
   const Library& core_lib = Library::Handle(Library::CoreLibrary());
-  Class& cls = Class::Handle(core_lib.LookupClass(cls_name));
+  // No ambiguity error expected: passing NULL.
+  Class& cls = Class::Handle(core_lib.LookupClass(cls_name, NULL));
   ASSERT(!cls.IsNull());
   // There are no parameterized error types, so no need to set type arguments.
   return Instance::New(cls);