Make TypedefMirror a direct descendant of TypeMirror.

sdk/lib/_internal/lib/mirrors_patch.dart

Index: sdk/lib/_internal/lib/mirrors_patch.dart
diff --git a/sdk/lib/_internal/lib/mirrors_patch.dart b/sdk/lib/_internal/lib/mirrors_patch.dart
index 47009848c8972607cc5cc8db586f282668da300d..271f54db7dd7df890cbd73d179fa68ca0a749c06 100644
--- a/sdk/lib/_internal/lib/mirrors_patch.dart
+++ b/sdk/lib/_internal/lib/mirrors_patch.dart
@@ -19,5 +19,11 @@ patch Future<MirrorSystem> mirrorSystemOf(SendPort port) {
 patch InstanceMirror reflect(Object reflectee) => js.reflect(reflectee);
 
 patch ClassMirror reflectClass(Type key) {
-  return js.reflectType(key).originalDeclaration;
+  TypeMirror tm = reflectType(key);
+  if (tm is ClassMirror) {
+    return (tm as ClassMirror).originalDeclaration;
+  }
+  throw new ArgumentError("$key does not denote a class");
 }
+
+patch TypeMirror reflectType(Type key) => js.reflectType(key);