dart2js: Use type-checks in later operations.

tests/compiler/dart2js/use_checks_test.dart

+// Copyright (c) 2014, 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';
+import "package:async_helper/async_helper.dart";
+import 'memory_compiler.dart';

[herhut, 2014/11/07 13:36:58]

Nit: Do you need the memory compiler here or would the mock do? See
for_in_test.dart for an example with less boiler plate code.

[floitsch, 2014/11/07 13:58:16]

Unfortunately the mock compiler doesn't work.

+
+const MEMORY_SOURCE_FILES = const {
+  'main.dart': '''
+main (x, y) {
+  if (x != null) {
+    if (y != null) {
+      // Forces x and y to be int-checked.
+      int a = x;
+      int b = y;
+      // Now we must be able to do a direct "+" operation in JS.
+      return x + y;
+    }
+  }
+}
+''',
+};
+
+main() {
+  var compiler = compilerFor(MEMORY_SOURCE_FILES,
+                             options: ['--enable-checked-mode']);
+  asyncTest(() => compiler.runCompiler(Uri.parse('memory:main.dart')).then((_) {
+    var element = compiler.mainApp.findExported('main');
+    var code = compiler.backend.assembleCode(element);
+    Expect.isTrue(code.contains('+'), code);
+  }));
+}