Reapply changes to Expando patching.

lib/compiler/implementation/lib/core_patch.dart

Index: lib/compiler/implementation/lib/core_patch.dart
diff --git a/lib/compiler/implementation/lib/core_patch.dart b/lib/compiler/implementation/lib/core_patch.dart
index 0f6c7d1e82e012b31b3291bda2da06fa51a89b85..4e22ea13ab51d931a8a572ffcbaa52b8af3d9d9f 100644
--- a/lib/compiler/implementation/lib/core_patch.dart
+++ b/lib/compiler/implementation/lib/core_patch.dart
@@ -14,3 +14,35 @@ patch class Object {
     throw new NoSuchMethodError(this, name, args);
   }
 }
+
+
+// Patch for Expando implementation.
+patch class Expando<T> {
+
+  patch T operator[](Object object) {
+    var values = Primitives.getProperty(object, _EXPANDO_PROPERTY_NAME);
+    return (values === null) ? null : Primitives.getProperty(values, _getKey());
+  }
+
+  patch void operator[]=(Object object, T value) {
+    var values = Primitives.getProperty(object, _EXPANDO_PROPERTY_NAME);
+    if (values === null) {
+      values = new Object();
+      Primitives.setProperty(object, _EXPANDO_PROPERTY_NAME, values);
+    }
+    Primitives.setProperty(values, _getKey(), value);
+  }
+
+  String _getKey() {
+    String key = Primitives.getProperty(this, _KEY_PROPERTY_NAME);
+    if (key === null) {
+      key = "expando\$key\$${_keyCount++}";
+      Primitives.setProperty(this, _KEY_PROPERTY_NAME, key);
+    }
+    return key;
+  }
+
+  static const String _KEY_PROPERTY_NAME = 'expando\$key';
+  static const String _EXPANDO_PROPERTY_NAME = 'expando\$values';
+  static int _keyCount = 0;
+}