Fix for issue 28854.

tests/standalone/regress_28854_2_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.
+//
+library regress;
+
+import 'dart:isolate';
+
+abstract class N<T> {
+
+}
+
+class J extends N<String> {
+  final String s;
+  J(this.s);
+}
+
+class K extends N<int> {
+  final int i;
+  K(this.i);
+}
+
+void isolate(SendPort port) {
+  port.send(new J("8" * 4));
+  for (int i=0; i<80000; i++) {
+    port.send(new K(i));
+  }
+  port.send('done');
+}
+
+main() async {
+  var recv = new RawReceivePort();
+  recv.handler = (k) {
+    if (k is J) print(k.s.length);
+    if (k is String) {
+      print(k);
+      recv.close();
+    }
+  };
+  var iso = await Isolate.spawn(isolate, recv.sendPort);
+}
+