Revert two Kernel changes that were causing test failures.

tests/language_2/async_await_foreign_test.dart

-// Copyright (c) 2017, 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 'dart:async';
-import 'package:expect/expect.dart';
-
-typedef Future<Null> Task();
-
-class ForeignFuture implements Future<Null> {
-  ForeignFuture(List<Task> tasks) {
-    tasks.forEach(_addTask);
-  }
-
-  Future<Null> _future;
-
-  void _addTask(Task task) {
-    _future = (_future == null) ? task() : _future.then((_) => task());
-  }
-
-  Future<S> then<S>(FutureOr<S> onValue(Null _), {Function onError}) {
-    assert(_future != null);
-    return _future.then((_) {
-      _future = null;
-      return onValue(null);
-    }, onError: (error, trace) {
-      _future = null;
-      if (onError != null) {
-        onError(error, trace);
-      }
-    });
-  }
-
-  Stream<Null> asStream() {
-    return new Stream.fromFuture(this);
-  }
-
-  Future<Null> catchError(onError, {bool test(Object error)}) {
-    print('Unimplemented catchError');
-    return null;
-  }
-
-  Future<Null> timeout(Duration timeLimit, {onTimeout()}) {
-    print('Unimplemented timeout');
-    return null;
-  }
-
-  Future<Null> whenComplete(action()) {
-    print('Unimplemented whenComplete');
-    return null;
-  }
-}
-
-var r1;
-
-Future<Null> hello() async {
-  r1 = 'hello';
-  throw new Exception('error');
-}
-
-Future<String> world() async {
-  try {
-    await new ForeignFuture([hello]);
-  } catch (e) {}
-  return 'world';
-}
-
-Future main() async {
-  var r2 = await world();
-  Expect.equals('hello', r1);
-  Expect.equals('world', r2);
-}