pkg/collection: mark fields in wrappers as final

pkg/collection/lib/wrappers.dart

Index: pkg/collection/lib/wrappers.dart
diff --git a/pkg/collection/lib/wrappers.dart b/pkg/collection/lib/wrappers.dart
index 1097c3ef17771e03486537e3129735e25543ff5d..b15a6bdaf3f477bd6431970e091f9671aa61ac23 100644
--- a/pkg/collection/lib/wrappers.dart
+++ b/pkg/collection/lib/wrappers.dart
@@ -26,7 +26,7 @@ part "src/unmodifiable_wrappers.dart";
  * iterable object.
  */
 class DelegatingIterable<E> implements Iterable<E> {
-  Iterable<E> _base;
+  final Iterable<E> _base;
 
   /**
    * Create a wrapper that forwards operations to [base].
@@ -295,7 +295,8 @@ class DelegatingQueue<E> extends DelegatingIterable<E> implements Queue<E> {
  * map object.
  */
 class DelegatingMap<K, V> implements Map<K, V> {
-  Map<K, V> _base;
+  final Map<K, V> _base;
+
   DelegatingMap(Map<K, V> base) : _base = base;
 
   V operator [](Object key) => _base[key];