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

runtime/vm/parser.cc

Index: runtime/vm/parser.cc
===================================================================
--- runtime/vm/parser.cc	(revision 25114)
+++ runtime/vm/parser.cc	(working copy)
@@ -1603,7 +1603,7 @@
     name = String::Concat(name, String::Handle(core_lib.private_key()));
     name = Symbols::New(name);
   }
-  return core_lib.LookupClass(name);
+  return core_lib.LookupClass(name, NULL);  // No ambiguity error expected.
 }
 
 
@@ -2843,8 +2843,7 @@
     ErrorMsg(method->name_pos, "constructor cannot be abstract");
   }
   if (method->has_const && method->IsConstructor()) {
-    Class& cls = Class::Handle(library_.LookupClass(members->class_name()));
-    cls.set_is_const();
+    current_class().set_is_const();
   }
 
   // Parse the formal parameters.