Implementation of null-aware operators.

tests/compiler/dart2js_extra/conditional_send_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.
 
+// SharedOptions=--enable-null-aware-operators
 import "package:expect/expect.dart";
 
-int count = 0;
+@NoInline() @AssumeDynamic()
+confuse(x) => x;
 
-void f([void f([x]) = f]) {
-  count++;
-  if (f != null) {
-    f(null);
-  }
+class A {
+  int x;
+  m() => "a";
 }
 
-main() {
-  f();
-  Expect.equals(2, count);
+main(args) {
+  var a = confuse(true) ? null : new A();
+  a?.x = 3;
+  Expect.throws(() => a.m());
 }