fix ListMixin _filter and sort to work in strong mode

sdk/lib/collection/list.dart

Index: sdk/lib/collection/list.dart
diff --git a/sdk/lib/collection/list.dart b/sdk/lib/collection/list.dart
index b81b8dd7c11fc2180336f577443e4a92fe3b66d7..b0c2cae45d9aa4387b735261cb5086c33517cf48 100644
--- a/sdk/lib/collection/list.dart
+++ b/sdk/lib/collection/list.dart
@@ -274,10 +274,8 @@ abstract class ListMixin<E> implements List<E> {
     _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];
@@ -308,11 +306,12 @@ abstract class ListMixin<E> implements List<E> {
   }
 
   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]) {