Fix bug in Future.forEach. Add tests.

sdk/lib/async/future.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;
 
 /// A type representing values that are either `Future<T>` or `T`.
 ///
 /// This class declaration is a public stand-in for an internal
 /// future-or-value generic type. References to this class are resolved to the
@@ skipping 433 matching lines @@
    * are discarded.
    *
    * Any error from [f], synchronous or asynchronous, will stop the iteration
    * and will be reported in the returned [Future].
    */
   static Future forEach<T>(Iterable<T> input, FutureOr f(T element)) {
     var iterator = input.iterator;
     return doWhile(() {
       if (!iterator.moveNext()) return false;
       var result = f(iterator.current);
-      if (result is Future<T>) return result.then(_kTrue);
+      if (result is Future) return result.then(_kTrue);
       return true;
     });
   }
 
   // Constant `true` function, used as callback by [forEach].
   static bool _kTrue(_) => true;
 
   /**
    * Performs an operation repeatedly until it returns `false`.
    *
@@ skipping 421 matching lines @@
   AsyncError replacement = Zone.current.errorCallback(error, stackTrace);
   if (replacement != null) {
     error = _nonNullError(replacement.error);
     stackTrace = replacement.stackTrace;
   }
   result._asyncCompleteError(error, stackTrace);
 }
 
 /** Helper function that converts `null` to a [NullThrownError]. */
 Object _nonNullError(Object error) => error ?? new NullThrownError();