Better boolean conversion tests.

tool/input_sdk/private/js_helper.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.
 
 library dart._js_helper;
 
 import 'dart:collection';
 
 import 'dart:_foreign_helper' show
     JS,
@@ skipping 849 matching lines @@
 
   SyncIterable(this._generator, this._args);
 
   // TODO(jmesserly): this should be [Symbol.iterator]() method. Unfortunately
   // we have no way of telling the compiler yet, so it will generate an extra
   // layer of indirection that wraps the SyncIterator.
   _jsIterator() => JS('', '#(...#)', _generator, _args);
 
   Iterator<E> get iterator => new SyncIterator<E>(_jsIterator());
 }
+
+class BooleanConversionAssertionError extends AssertionError {
+  toString() => 'Failed assertion: boolean expression must not be null';
+}