Reapply 'Add runtimeType() to Object which returns canonicalized instances of Type'.

lib/core/object.dart

Index: lib/core/object.dart
diff --git a/lib/core/object.dart b/lib/core/object.dart
index 3c94515e68f752823c3c8881c725ec64d38cea7f..9bf5bd0cf77819608a971c4be89e41d6864b0f78 100644
--- a/lib/core/object.dart
+++ b/lib/core/object.dart
@@ -13,8 +13,12 @@ class Object {
   external String toString();
 
   external void noSuchMethod(String name, List args);
+
+  external Type runtimeType();
 }
 
+abstract class Type {}
+
 /**
  * Check whether two references are to the same object.
  */