Fix unnecessary use of this.length in ListMixin.

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 92b1110b192500795207b14ccb9491d57c7b696c..3ce8ee27df06d8d4cd9950b05203e88caa8b6fa8 100644
--- a/sdk/lib/collection/list.dart
+++ b/sdk/lib/collection/list.dart
@@ -89,7 +89,7 @@ abstract class ListMixin<E> implements List<E> {
 
   bool contains(Object element) {
     int length = this.length;
-    for (int i = 0; i < this.length; i++) {
+    for (int i = 0; i < length; i++) {
       if (this[i] == element) return true;
       if (length != this.length) {
         throw new ConcurrentModificationError(this);