dart2js cps: Use JS semantics for logical operators in the tree IR.

tests/language/conditional_rewrite_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.
+
+import "package:expect/expect.dart";
+
+// Test that dart2js does not rewrite conditional into logical operators
+// in cases where it changes which falsy value is returned.
+
+posFalse(x, y) => x != null ? y : false;
+negFalse(x, y) => x != null ? !y : false;
+posNull(x, y) => x != null ? y : null;
+negNull(x, y) => x != null ? !y : null;
+
+main() {
+  bool isCheckedMode = false;
+  assert((isCheckedMode = true));
+
+  Expect.equals(false, posFalse(null, false));
+  Expect.equals(false, negFalse(null, false));
+  Expect.equals(null, posNull(null, false));
+  Expect.equals(null, negNull(null, false));
+
+  Expect.equals(false, posFalse(null, true));
+  Expect.equals(false, negFalse(null, true));
+  Expect.equals(null, posNull(null, true));
+  Expect.equals(null, negNull(null, true));
+
+  Expect.equals(false, posFalse([], false));
+  Expect.equals(true, negFalse([], false));
+  Expect.equals(false, posNull([], false));
+  Expect.equals(true, negNull([], false));
+
+  Expect.equals(true, posFalse([], true));
+  Expect.equals(false, negFalse([], true));
+  Expect.equals(true, posNull([], true));
+  Expect.equals(false, negNull([], true));
+
+  if (!isCheckedMode) {
+    Expect.equals(null, posFalse([], null));
+    Expect.equals(true, negFalse([], null));
+    Expect.equals(null, posNull([], null));
+    Expect.equals(true, negNull([], null));
+
+    var y = {};
+    Expect.identical(y, posFalse([], y));
+    Expect.equals(true, negFalse([], y));
+    Expect.identical(y, posNull([], y));
+    Expect.equals(true, negNull([], y));
+  }
+}