Remove generic typedefs that dart2js chokes on.

dart/sdk/lib/async/future_impl.dart

 // Copyright (c) 2012, 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.
 
 // part of dart.async;
 
 deprecatedFutureValue(_FutureImpl future) =>
   future._isComplete ? future._resultOrListeners : null;
 
 
@@ skipping 306 matching lines @@
 /** The onValue and onError handlers return either a value or a future */
 typedef dynamic _FutureOnValue<T>(T value);
 typedef dynamic _FutureOnError(AsyncError error);
 /** Test used by [Future.catchError] to handle skip some errors. */
 typedef bool _FutureErrorTest(var error);
 /** Used by [WhenFuture]. */
 typedef void _FutureAction();
 
 /** Future returned by [Future.then] with no [:onError:] parameter. */
 class _ThenFuture<S, T> extends _TransformFuture<S, T> {
-  final _FutureOnValue<S> _onValue;
+  // TODO(ahe): Restore type when feature is implemented in dart2js
+  // checked mode.
+  final /* _FutureOnValue<S> */ _onValue;
 
   _ThenFuture(this._onValue);
 
   _sendValue(S value) {
     assert(_onValue != null);
     var result;
     try {
       result = _onValue(value);
     } catch (e, s) {
       _setError(new AsyncError(e, s));
@@ skipping 113 matching lines @@
   Future catchError(function(AsyncError error), {bool test(var error)}) {
     return _future.catchError(function, test: test);
   }
 
   Future whenComplete(void action()) {
     return _future.whenComplete(action);
   }
 
   Stream<T> asStream() => new Stream.fromFuture(this);
 }