Fix runtime strong mode error in ListMixin

pkg/dev_compiler/test/codegen/lib/collection/list_test.dart

Index: pkg/dev_compiler/test/codegen/lib/collection/list_test.dart
diff --git a/pkg/dev_compiler/test/codegen/lib/collection/list_test.dart b/pkg/dev_compiler/test/codegen/lib/collection/list_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..76cd515a92deef17a0305a257a0b7e934d2ee856
--- /dev/null
+++ b/pkg/dev_compiler/test/codegen/lib/collection/list_test.dart
@@ -0,0 +1,40 @@
+// Copyright (c) 2013, 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 MyList<E> extends Object with ListMixin<E> implements List<E> {
+  List<E> _list;
+
+  MyList(List<E> this._list);
+
+  int get length => _list.length;
+
+  void set length(int x) {
+    _list.length = x;
+  }
+
+  E operator[](int idx) => _list[idx];
+
+  void operator[]=(int idx, E value) {
+    _list[idx] = value;
+  }
+}
+
+void testRetainWhere() {
+  List<int> list = <int>[1, 2, 3];
+  list.retainWhere((x) => x % 2 == 0);
+  Expect.equals(1, list.length);
+  Expect.equals(2, list.first);
+
+  list = new MyList<int>([1, 2, 3]);

[vsm, 2016/12/01 23:33:54]

FYI: The strong mode error only kicked in here prior to the fix - native array
was fine.

+  list.retainWhere((x) => x % 2 == 0);
+  Expect.equals(1, list.length);
+  Expect.equals(2, list.first);
+}
+
+void main() {
+  testRetainWhere();
+}