Make TypedefMirror a direct descendant of TypeMirror.

runtime/lib/mirrors.cc

Index: runtime/lib/mirrors.cc
diff --git a/runtime/lib/mirrors.cc b/runtime/lib/mirrors.cc
index cb388be55538b35df8985a9d95d671cf880796a3..dc6808b8b8974895958d52688cf6f22a8a88563c 100644
--- a/runtime/lib/mirrors.cc
+++ b/runtime/lib/mirrors.cc
@@ -839,7 +839,10 @@ DEFINE_NATIVE_ENTRY(Mirrors_makeLocalClassMirror, 1) {
   ASSERT(type.IsFinalized());
   ASSERT(type.HasResolvedTypeClass());
   const Class& cls = Class::Handle(type.type_class());
-  if (cls.IsDynamicClass() || cls.IsVoidClass()) {
+  ASSERT(!cls.IsNull());
+  if (cls.IsDynamicClass() ||
+      cls.IsVoidClass() ||
+      (cls.IsSignatureClass() && !cls.IsCanonicalSignatureClass())) {
     Exceptions::ThrowArgumentError(type);
     UNREACHABLE();
   }