Fix bug in Future.forEach. Add tests.

sdk/lib/async/future.dart

Index: sdk/lib/async/future.dart
diff --git a/sdk/lib/async/future.dart b/sdk/lib/async/future.dart
index ea999b1299dcb07c4e0c657529b4218960534194..0fceb545e4ad63e77e930175e6fe72ca240b99ab 100644
--- a/sdk/lib/async/future.dart
+++ b/sdk/lib/async/future.dart
@@ -451,7 +451,7 @@ abstract class Future<T> {
     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;
     });
   }