Implement the new '??' operator in analyzer.

tests/language/if_null_precedence_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 '??' binds tighter than '?:' and less tightly than '||'.
+
+// SharedOptions=--enable-null-aware-operators
+
+import "package:expect/expect.dart";
+
+assertionError(e) => e is AssertionError;
+
+// Determine whether the VM is running in checked mode.
+bool get checkedMode {
+  try {
+    var x = 'foo';
+    int y = x;
+    return false;
+  } catch (_) {
+    return true;
+  }
+}
+
+main() {
+  // Make sure the "none" test fails if "??" is not implemented.  This makes
+  // status files easier to maintain.
+  var _ = null ?? null;
+
+  // "a ?? b ?? c" should be legal, and should evaluate to the first non-null
+  // value (or null if there are no non-null values).
+  Expect.equals(1, 1 ?? 2 ?? 3); /// 01: ok
+  Expect.equals(2, null ?? 2 ?? 3); /// 02: ok
+  Expect.equals(3, null ?? null ?? 3); /// 03: ok
+  Expect.equals(null, null ?? null ?? null); /// 04: ok
+
+  // "a ?? b ? c : d" should parse as "(a ?? b) ? c : d", therefore provided
+  // that a is true, b need not be a bool.  An incorrect parse of
+  // "a ?? (b ? c : d)" would require b to be a bool to avoid a static type
+  // warning.
+  Expect.equals(2, true ?? 1 ? 2 : 3); /// 05: ok
+
+  // "a ?? b || c" should parse as "a ?? (b || c)", therefore it's a static
+  // type warning if b doesn't have type bool.  An incorrect parse of
+  // "(a ?? b) || c" would allow b to have any type provided that a is bool.
+  Expect.equals(false, false ?? 1 || true); /// 06: static type warning
+
+  // "a || b ?? c" should parse as "(a || b) ?? c", therefore it is a static
+  // type warning if b doesn't have type bool.  An incorrect parse of
+  // "a || (b ?? c)" would allow b to have any type provided that c is bool.
+  if (checkedMode) {
+    Expect.throws(() => false || 1 ?? true, assertionError); /// 07: static type warning
+  } else {
+    Expect.equals(false, false || 1 ?? true);                /// 07: continued
+  }
+
+  if (checkedMode) {
+    // An incorrect parse of "a || (b ?? c)" would result in no checked-mode
+    // error.
+    Expect.throws(() => false || null ?? true, assertionError); /// 08: ok
+  } else {
+    // An incorrect parse of "a || (b ?? c)" would result in c being evaluated.
+    int i = 0;                                                  /// 08: continued
+    Expect.equals(false, false || null ?? i++ == 0);            /// 08: continued
+    Expect.equals(0, i);                                        /// 08: continued
+  }
+}