Add experimental expando support.

lib/compiler/implementation/lib/mock.dart

Index: lib/compiler/implementation/lib/mock.dart
diff --git a/lib/compiler/implementation/lib/mock.dart b/lib/compiler/implementation/lib/mock.dart
index 7950f9d2df380d525e558aae2b1b8c0c1bcfdc1e..89e26be7fe34929bedd1de454fa3c135ea42d88c 100644
--- a/lib/compiler/implementation/lib/mock.dart
+++ b/lib/compiler/implementation/lib/mock.dart
@@ -48,3 +48,43 @@ class _InternalByteArray {
 void exit(int exitCode) {
   throw new UnsupportedOperationException("exit($exitCode)");
 }
+
+class _Expando<T> implements Expando<T> {
+
+  final String name;
+
+  const _Expando([this.name]);
+
+  T operator[](Object object) {
+    var values = Primitives.getProperty(object, EXPANDO_PROPERTY_NAME);
+    return (values === null) ? null : Primitives.getProperty(values, _getKey());
+  }
+
+  void operator[]=(Object object, T value) {
+    var values = Primitives.getProperty(object, EXPANDO_PROPERTY_NAME);
+    if (values === null) {
+      values = new Object();
+      Primitives.setProperty(object, EXPANDO_PROPERTY_NAME, values);
+    }
+    Primitives.setProperty(values, _getKey(), value);
+  }
+
+  String toString() {
+    String key = _getKey();
+    return (name === null) ? "Expando:${key}" : "Expando:${name}@${key}";
+  }
+
+  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 final String KEY_PROPERTY_NAME = 'expando\$key';
+  static final String EXPANDO_PROPERTY_NAME = 'expando\$values';
+  static int keyCount = 0;
+
+}