Lazy implementation of final variables.

tests/language/lazy_final3_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();
+var x2 = foo2();
+var x3 = foo3();
+var x4 = foo4();
+var x5 = foo5();
+final x6 = foo6();
+var x7 = x7 + 1;
+
+foo() { throw "interrupt initialization"; }
+foo2() { x2 = 499; throw "interrupt initialization"; }
+foo3() => x3 + 1;
+foo4() {
+  x4 = 498;
+  x4 = x4 + 1;
+  return x4;
+}
+foo5() {
+  x5 = 498;
+  x5 = x5 + 1;
+}
+foo6() {
+  try {
+    return x5 + 1;
+  } catch (e) {
+    return 499;
+  }
+}
+
+fib(x) {
+  if (x is! int) return 0;
+  if (x < 2) return x;
+  return fib(x - 1) + fib(x - 2);
+}
+
+main() {
+  Expect.throws(() => fib(x), (e) => e == "interrupt initialization");
+  Expect.equals(null, x);
+
+  Expect.throws(() => fib(x2), (e) => e == "interrupt initialization");
+  Expect.equals(499, x2);
+
+  Expect.throws(() => fib(x3), (e) => e is RuntimeError);
+  Expect.equals(null, x3);
+
+  Expect.equals(499, x4);
+
+  Expect.equals(null, x5);
+
+  Expect.equals(499, x6);
+
+  Expect.throws(() => fib(x7), (e) => e is RuntimeError);
+}