Implement const expressions for local variables

tests/language/const_locals_test.dart

+// 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.
+
+// Test of compile time constant local variables.
+
+const N = 8;
+
+class ConstFoo {
+  final x;
+  const ConstFoo(this.x);
+}
+
+main() {
+  const MIN = 2 - 1;
+  const MAX = N * 2;
+  const MASK = (1 << (MAX - MIN + 1)) - 1;  // 65535.
+  Expect.equals(1, MIN);
+  Expect.equals(16, MAX);
+  Expect.equals(65535, MASK);
+  const s = 'MIN = $MIN  MAX = $MAX  MASK = $MASK';
+  Expect.isTrue(s === 'MIN = $MIN  MAX = $MAX  MASK = $MASK');
+  Expect.equals("MIN = 1  MAX = 16  MASK = 65535", s);
+  var cf1 = const ConstFoo(MASK);
+  var cf2 = const ConstFoo(s);
+  var cf3 = const ConstFoo('MIN = $MIN  MAX = $MAX  MASK = $MASK');
+  Expect.isTrue(cf2 === cf3);
+  Expect.isFalse(cf2 === cf1);
+}
+
+
+