Implement VariableMirror.isConst in the VM and ParameterMirror.isConst in dart2js.

tests/lib/mirrors/variable_is_const_test.dart

+// Copyright (c) 2013, 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.
+
+library test.variable_is_const;
+
+import 'dart:mirrors';
+
+import 'package:expect/expect.dart';
+
+class Class {
+  const /// 01: compile-time error
+      int instanceWouldBeConst = 1;
+  var instanceNonConst = 2;
+
+  static const staticConst = 3;
+  static var staticNonConst = 4;
+}
+
+const topLevelConst = 5;
+var topLevelNonConst = 6;
+
+main() {
+  bool isConst(m, Symbol s) => (m.declarations[s] as VariableMirror).isConst;
+
+  ClassMirror cm = reflectClass(Class);
+  Expect.isFalse(isConst(cm, #instanceWouldBeConst));
+  Expect.isFalse(isConst(cm, #instanceNonConst));
+  Expect.isTrue(isConst(cm, #staticConst));
+  Expect.isFalse(isConst(cm, #staticNonConst));
+
+  LibraryMirror lm = cm.owner;
+  Expect.isTrue(isConst(lm, #topLevelConst));
+  Expect.isFalse(isConst(lm, #topLevelNonConst));
+}