Implement the new '??=' operator in analyzer.

tests/language/if_null_assignment_helper.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.
+
+// Library used by if_null_assignment_behavior_test.dart, which
+// imports it using the prefix "h.".
+
+library lib;
+
+import "package:expect/expect.dart";
+
+List<String> operations = [];
+
+var xGetValue = null;
+
+get x {
+  operations.add('h.x');
+  var tmp = xGetValue;
+  xGetValue = null;
+  return tmp;
+}
+
+void set x(value) {
+  operations.add('h.x=$value');
+}
+
+class C {
+  static var xGetValue = null;
+
+  static get x {
+    operations.add('h.C.x');
+    var tmp = xGetValue;
+    xGetValue = null;
+    return tmp;
+  }
+
+  static void set x(value) {
+    operations.add('h.C.x=$value');
+  }
+}