Make IterableMixinWorkaround using classes return correctly typed Iterables.

sdk/lib/internal/list.dart

Index: sdk/lib/internal/list.dart
diff --git a/sdk/lib/internal/list.dart b/sdk/lib/internal/list.dart
index 473a87a622cc8b947c3b208c4928a84ef18fa85f..43e7e6a730efe8019b5c8fe5227d57734660330e 100644
--- a/sdk/lib/internal/list.dart
+++ b/sdk/lib/internal/list.dart
@@ -295,6 +295,8 @@ class ListMapView<E> implements Map<int, E> {
   void addAll(Map<int, E> other) {
     throw new UnsupportedError("Cannot modify an unmodifiable map");
   }
+
+  String toString() => Maps.mapToString(this);
 }
 
 class ReversedListIterable<E> extends ListIterable<E> {