fix ListMixin _filter and sort to work in strong mode

sdk/lib/collection/list.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.
 
 part of dart.collection;
 
 /**
  * Abstract implementation of a list.
  *
  * `ListBase` can be used as a base class for implementing the `List` interface.
@@ skipping 256 matching lines @@
   }
 
   void removeWhere(bool test(E element)) {
     _filter(this, test, false);
   }
 
   void retainWhere(bool test(E element)) {
     _filter(this, test, true);
   }
 
-  static void _filter(List source,
-                      bool test(var element),
-                      bool retainMatching) {
-    List retained = [];
+  void _filter(List source, bool test(var element), bool retainMatching) {

[Jennifer Messerly, 2017/02/28 18:36:34]

I also made the change you suggested here:
https://codereview.chromium.org/2698353003/diff/80001/sdk/lib/collection/list...

+    List<E> retained = <E>[];
     int length = source.length;
     for (int i = 0; i < length; i++) {
       var element = source[i];
       if (test(element) == retainMatching) {
         retained.add(element);
       }
       if (length != source.length) {
         throw new ConcurrentModificationError(source);
       }
     }
@@ skipping 10 matching lines @@
   E removeLast() {
     if (length == 0) {
       throw IterableElementError.noElement();
     }
     E result = this[length - 1];
     length--;
     return result;
   }
 
   void sort([int compare(E a, E b)]) {
-    if (compare == null) {
-      Sort.sort(this, Comparable.compare);
-    } else {
-      Sort.sort(this, compare);
-    }
+    Sort.sort(this, compare ?? _compareAny);
+  }
+
+  static int _compareAny(a, b) {
+    // In strong mode this requires a cast to ensure `a` and `b` are Comparable.

[Lasse Reichstein Nielsen, 2017/02/28 07:05:56]

Confusing comment - I don't see any cast?

[Jennifer Messerly, 2017/02/28 18:36:35]

Ah thanks! will clarify. The cast is added implicitly.

+    return Comparable.compare(a, b);
   }
 
   void shuffle([Random random]) {
     if (random == null) random = new Random();
     int length = this.length;
     while (length > 1) {
       int pos = random.nextInt(length);
       length -= 1;
       var tmp = this[length];
       this[length] = this[pos];
@@ skipping 181 matching lines @@
       for (E element in iterable) {
         this[index++] = element;
       }
     }
   }
 
   Iterable<E> get reversed => new ReversedListIterable<E>(this);
 
   String toString() => IterableBase.iterableToFullString(this, '[', ']');
 }