Implement the new '??' operator in analyzer.

tests/language/if_null_behavior_test.dart

Index: tests/language/if_null_behavior_test.dart
diff --git a/tests/language/if_null_behavior_test.dart b/tests/language/if_null_behavior_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..4fcbfa791053b172cab3453314e5b1c89b05f131
--- /dev/null
+++ b/tests/language/if_null_behavior_test.dart
@@ -0,0 +1,55 @@
+// 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).  The
+// static type of e is the least upper bound of the static type of e1 and the
+// static type of e2.
+
+// SharedOptions=--enable-null-aware-operators
+
+import "package:expect/expect.dart";
+
+class A {
+  final String a;
+  A(this.a);
+}
+
+class B extends A {
+  B(String v) : b = v, super(v);
+  final String b;
+}
+
+class C extends A {
+  C(String v) : c = v, super(v);
+  final String c;
+}
+
+B nullB() => null;
+C nullC() => null;
+
+noMethod(e) => e is NoSuchMethodError;
+
+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 ?? 2); /// 01: ok
+  Expect.equals(1, 1 ?? null); /// 02: ok
+  Expect.equals(2, null ?? 2); /// 03: ok
+  Expect.equals(null, null ?? null); /// 04: ok
+  Expect.equals('B', (new B('B') ?? new C('C')).a); /// 05: ok
+  Expect.equals('B', (new B('B') ?? new C('C')).b); /// 06: static type warning
+  Expect.throws(() => (new B('B') ?? new C('C')).c, noMethod); /// 07: static type warning
+  Expect.equals('B', (new B('B') ?? nullC()).a); /// 08: ok
+  Expect.equals('B', (new B('B') ?? nullC()).b); /// 09: static type warning
+  Expect.throws(() => (new B('B') ?? nullC()).c, noMethod); /// 10: static type warning
+  Expect.equals('C', (nullB() ?? new C('C')).a); /// 11: ok
+  Expect.throws(() => (nullB() ?? new C('C')).b, noMethod); /// 12: static type warning
+  Expect.equals('C', (nullB() ?? new C('C')).c); /// 13: static type warning
+  Expect.throws(() => (nullB() ?? nullC()).a, noMethod); /// 14: ok
+  Expect.throws(() => (nullB() ?? nullC()).b, noMethod); /// 15: static type warning
+  Expect.throws(() => (nullB() ?? nullC()).c, noMethod); /// 16: static type warning
+}