Ensure the type inferrer does not infer types for runtime helpers used by the backend.

tests/compiler/dart2js_native/inference_of_helper_methods_test.dart

+// Copyright (c) 2013, 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 'dart:_js_helper' show intTypeCheck;
+
+bool get inCheckedMode {
+  try {
+    String a = 42;
+  } on TypeError catch (e) {
+    return true;
+  }
+  return false;
+}
+
+main() {
+  var a = [];
+  a.add(42);
+  a.add('foo');
+  // By calling direclty [intTypeCheck] with an int, we're making the
+  // type inferrer infer that the parameter type of [intTypeCheck] is
+  // always an int, and therefore the method will be compiled to
+  // never throw. So when the backend actually uses the helper for
+  // implementing checked mode semantics (like in the check below),
+  // the check won't fail at runtime.
+  intTypeCheck(42);
+  if (inCheckedMode) {
+    int value;
+    Expect.throws(() => value = a[1], (e) => e is TypeError);
+  }
+}