Fix for issue 4921: ad the generic type info to a List when we create it.

tests/language/list_is_test.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.
+
+class A<T> {
+  bar() => new List<T>();
+}
+
+main() {
+  check(new List(), true, true, true);
+  check(new List<int>(), true, true, false);
+  check(new A().bar(), true, true, true);
+  check(new A<double>().bar(), true, false, true);
+  check(new Object(), false, false, false);
+}
+
+check(val, expectList, expectListInt, expectListDouble) {
+  Expect.equals(expectList, val is List);
+  Expect.equals(expectListInt, val is List<int>);
+  Expect.equals(expectListDouble, val is List<double>);
+}