Implementation of null-aware operators.

tests/compiler/dart2js_extra/if_null_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";
+
+@NoInline() @AssumeDynamic()
+confuse(x) => x;
+
+main(args) {
+  var x = new A();
+  var y;
+
+  // Checks that inference doesn't incorrectly treat this as a normal
+  // assignment (where only B is a possible value after the assignment).
+  var c = x ??= new B();
+  var z = x;
+  Expect.equals('a', x.m());
+  Expect.equals('a', z.m());
+  Expect.equals('a', c.m());
+  if (confuse(true)) y = x;
+  Expect.equals('a', y.m());
+
+  // Similar test, within fields.
+  new C();
+  new D();
+}
+
+class A { m() => 'a'; }
+class B { m() => 'b'; }
+
+class C {
+  var y;
+  C() {
+    y = new A();
+    var c = y ??= new B();
+    Expect.equals('a', y.m());
+    Expect.equals('a', c.m());
+  }
+}
+
+class D {
+  var y;
+  D() {
+    this.y = new A();
+    var c = this.y ??= new B();
+    Expect.equals('a', y.m());
+    Expect.equals('a', c.m());
+  }
+}