Implement the new '??' operator in analyzer.

tests/language/if_null_evaluation_order_test.dart

+// Copyright (c) 2015, 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.
+
+// Evaluation of an if-null expresion e of the form e1 ?? e2 is equivalent to
+// the evaluation of the expression ((x) => x == null ? e2 : x)(e1).
+//
+// Therefore, e1 should be evaluated first; if it is non-null, e2 should not
+// be evaluated.
+
+// SharedOptions=--enable-null-aware-operators
+
+import "package:expect/expect.dart";
+
+void bad() {
+  throw new Exception();
+}
+
+bool firstExecuted = false;
+
+first() {
+  firstExecuted = true;
+  return null;
+}
+
+second() {
+  Expect.isTrue(firstExecuted);
+  return 2;
+}
+
+main() {
+  // Make sure the "none" test fails if "??" is not implemented.  This makes
+  // status files easier to maintain.
+  var _ = null ?? null;
+
+  Expect.equals(1, 1 ?? bad()); /// 01: ok
+  Expect.equals(2, first() ?? second()); /// 02: ok
+}