Implement constant maps.

frog/leg/lib/constant_map.dart

Index: frog/leg/lib/constant_map.dart
diff --git a/frog/leg/lib/constant_map.dart b/frog/leg/lib/constant_map.dart
new file mode 100644
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--- /dev/null
+++ b/frog/leg/lib/constant_map.dart
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+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// This class has no constructor. This is on purpose since the instantiation
+// is shortcut by the compiler.
+class ConstantMap<V> implements Map<String, V> {
+  final int length;
+  // A constant map is backed by a JavaScript object.
+  final _jsObject;
+  final List<String> _keys;
+
+  bool containsValue(V needle) {
+    return getValues().some((V value) => value == needle);
+  }
+
+  bool containsKey(String key) {
+    if (key == '__proto__') return false;
+    return jsHasOwnProperty(_jsObject, key);
+  }
+
+  V operator [](String key) {
+    if (!containsKey(key)) return null;
+    return jsPropertyAccess(_jsObject, key);
+  }
+
+  void forEach(void f(String key, V value)) {
+    _keys.forEach((String key) => f(key, this[key]));
+  }
+
+  Collection<String> getKeys() => _keys;
+
+  Collection<V> getValues() {
+    List<V> result = <V>[];
+    _keys.forEach((String key) => result.add(this[key]));
+    return result;
+  }
+
+  bool isEmpty() => length == 0;
+
+  String toString() => Maps.mapToString(this);
+
+  _throwImmutable() {
+    throw const IllegalAccessException();
+  }
+  void operator []=(String key, V val) => _throwImmutable();
+  V putIfAbsent(String key, V ifAbsent()) => _throwImmutable();
+  V remove(String key) => _throwImmutable();
+  void clear() => _throwImmutable();
+}