Lazy implementation of final variables.

tests/language/lazy_final_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.
+
+final x = foo();
+final y = y2(y3);
+final y2 = incrementCreator();
+final y3 = fib(5);
+
+foo() => 499;
+
+incrementCreator() => (x) => x + 1;
+fib(x) {
+  if (x < 2) return x;
+  return fib(x - 1) + fib(x - 2);
+}
+
+var count = 0;
+sideEffect() => count++;
+
+final t = sideEffect();
+final t2 = sideEffect();
+
+class A {
+  static final a = toto();
+  static final b = b2(b3);
+  static final b2 = decrementCreator();
+  static final b3 = fact(5);
+
+  static toto() => 666;
+
+  static decrementCreator() => (x) => x - 1;
+  static fact(x) {
+    if (x <= 1) return x;
+    return x * fact(x - 1);
+  }
+}
+
+main() {
+  Expect.equals(499, x);
+  Expect.equals(6, y);
+  Expect.equals(666, A.a);
+  Expect.equals(119, A.b);
+  Expect.equals(0, t);
+  t2 = 499;
+  Expect.equals(499, t2);
+  Expect.equals(1, count);
+}