Fix type error in js_dart2js.dart caught by DDC.

tests/lib_strong/collection/list_test.dart

 // 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];
+  E operator [](int idx) => _list[idx];
 
-  void operator[]=(int idx, E value) {
+  void operator []=(int idx, E value) {
     _list[idx] = value;
   }
 }
 
+class MyNoSuchMethodList<E> extends Object
+    with ListMixin<E>
+    implements List<E> {
+  List<E> _list;
+
+  MyNoSuchMethodList(List<E> this._list);
+
+  noSuchMethod(Invocation invocation) {
+    if (invocation.memberName == #length) {
+      if (invocation.isGetter) return _list.length;
+      if (invocation.isSetter) {
+        _list.length = invocation.positionalArguments.first;
+        return;
+      }
+      return super.noSuchMethod(invocation);
+    }
+    if (invocation.memberName == new Symbol("[]") &&
+        invocation.positionalArguments.length == 1) {
+      return _list[invocation.positionalArguments.first];
+    }
+    if (invocation.memberName == new Symbol("[]=") &&
+        invocation.positionalArguments.length == 2) {
+      _list[invocation.positionalArguments.first] =
+          invocation.positionalArguments[1];
+      return;
+    }
+    return super.noSuchMethod(invocation);
+  }
+}
+
 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]);
   list.retainWhere((x) => x % 2 == 0);
   Expect.equals(1, list.length);
   Expect.equals(2, list.first);
+
+  list = new MyNoSuchMethodList<int>([1, 2, 3]);
+  list.retainWhere((x) => x % 2 == 0);
+  Expect.equals(1, list.length);
+  Expect.equals(2, list.first);
+
+  // Equivalent tests where the type of the List is known statically.
+  {
+    var l = new MyList<int>([1, 2, 3]);
+    l.retainWhere((x) => x % 2 == 0);
+    Expect.equals(1, l.length);
+    Expect.equals(2, l.first);
+  }
+
+  {
+    var l = new MyNoSuchMethodList<int>([1, 2, 3]);
+    l.retainWhere((x) => x % 2 == 0);
+    Expect.equals(1, l.length);
+    Expect.equals(2, l.first);
+  }
 }
 
 void main() {
   testRetainWhere();
 }