Reapply changes to Expando patching.

lib/core/expando.dart

Index: lib/core/expando.dart
diff --git a/lib/core/expando.dart b/lib/core/expando.dart
index d537659146000ba03d5c339cd2f40adb432fb19b..901cee5c29ed24d621ed3d940031bd33c43c5745 100644
--- a/lib/core/expando.dart
+++ b/lib/core/expando.dart
@@ -5,7 +5,7 @@
 /**
  * An [Expando] allows adding new properties to objects.
  */
-interface Expando<T> default ExpandoImplementation<T> {
+class Expando<T> {
 
   /**
    * The name of the this [Expando] as passed to the constructor. If
@@ -19,20 +19,25 @@ interface Expando<T> default ExpandoImplementation<T> {
    * [Expando]s with the same name yields two [Expando]s that work on
    * different properties of the objects they are used on.
    */
-  const Expando([String name]);
+  const Expando([String this.name]);
+
+  /**
+   * Expando toString method override.
+   */
+  String toString() => "Expando:$name";
 
   /**
    * Gets the value of this [Expando]'s property on the given
    * object. If the object hasn't been expanded, the method returns
    * [null].
    */
-  T operator [](Object object);
+  external T operator [](Object object);
 
   /**
    * Sets the value of this [Expando]'s property on the given
    * object. Properties can effectively be removed again by setting
    * their value to null.
    */
-  void operator []=(Object object, T value);
+  external void operator []=(Object object, T value);
 
 }