Fix isAssignableTo in VM mirrors to use subtype (<:) rather than moreSpecificThan (<<).

runtime/lib/mirrors.cc

Index: runtime/lib/mirrors.cc
diff --git a/runtime/lib/mirrors.cc b/runtime/lib/mirrors.cc
index d90f69cef57b4664fac4efe22636f11044caef34..a7c11d77a4271ed4296ebc6144239f181a824b5a 100644
--- a/runtime/lib/mirrors.cc
+++ b/runtime/lib/mirrors.cc
@@ -2072,11 +2072,5 @@ DEFINE_NATIVE_ENTRY(TypeMirror_subtypeTest, 2) {
   return Bool::Get(a.IsSubtypeOf(b, NULL)).raw();
 }
 
-DEFINE_NATIVE_ENTRY(TypeMirror_moreSpecificTest, 2) {
-  GET_NON_NULL_NATIVE_ARGUMENT(AbstractType, a, arguments->NativeArgAt(0));
-  GET_NON_NULL_NATIVE_ARGUMENT(AbstractType, b, arguments->NativeArgAt(1));
-  return Bool::Get(a.IsMoreSpecificThan(b, NULL)).raw();
-}
-
 
 }  // namespace dart