Add experimental expando support.

lib/compiler/implementation/lib/mockimpl.dart

Index: lib/compiler/implementation/lib/mockimpl.dart
diff --git a/lib/compiler/implementation/lib/mockimpl.dart b/lib/compiler/implementation/lib/mockimpl.dart
index 08648b963210d8818313dab8ad876dd86f21a81e..d10ca5a15d489c3f1f3edf4dd590bab06c1e4b82 100644
--- a/lib/compiler/implementation/lib/mockimpl.dart
+++ b/lib/compiler/implementation/lib/mockimpl.dart
@@ -377,3 +377,27 @@ class ListFactory<E> {
     return result;
   }
 }
+
+class ExpandoImplementation<T> implements Expando<T> {
+
+  final String name;
+  final String _key;
+
+  ExpandoImplementation([this.name]) : _key = _generateKey();
+
+  T operator[](Object object) {
+    return Primitives.getProperty(object, _key);
+  }
+
+  void operator[]=(Object object, T value) {
+    Primitives.setProperty(object, _key, value);
+  }
+
+  String toString() {
+    return (name === null) ? "Expando:${_key}" : "Expando:${name}@${_key}";
+  }
+
+  static int _keyCount = 0;
+  static String _generateKey() => "\$key\$${_keyCount++}";
+
+}