DDC fix for foreign futures

tests/language_strong/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);
+}