Implementation of null-aware operators.

tests/language/this_conditional_operator_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.
+
+// Verify that the ?. operator cannot be used for forwarding "this"
+// constructors.
+
+// SharedOptions=--enable-null-aware-operators
+
+class B {
+  B();
+  B.namedConstructor();
+  var field = 1;
+  method() => 1;
+
+  B.forward()
+    : this?.namedConstructor() /// 01: compile-time error
+  ;
+
+  test() {
+    this?.field = 1;
+    this?.field += 1;
+    this?.field;
+    this?.method();
+  }
+}
+
+main() {
+  new B.forward().test();
+}