Migrate test block 11 to Dart 2.0.

tests/corelib/iterable_length_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.
-
-import "dart:collection";
-import "package:expect/expect.dart";
-
-class A extends IterableBase {
-  int count;
-  A(this.count);
-
-  Iterator get iterator {
-    return new AIterator(count);
-  }
-}
-
-class AIterator implements Iterator {
-  int _count;
-  int _current;
-
-  AIterator(this._count);
-
-  bool moveNext() {
-    if (_count > 0) {
-      _current = _count;
-      _count--;
-      return true;
-    }
-    _current = null;
-    return false;
-  }
-
-  get current => _current;
-}
-
-main() {
-  var a = new A(10);
-  Expect.equals(10, a.length);
-  a = new A(0);
-  Expect.equals(0, a.length);
-  a = new A(5);
-  Expect.equals(5, a.map((e) => e + 1).length);
-  Expect.equals(3, a.where((e) => e >= 3).length);
-}