Revert "Remove deprecated mappedBy."

sdk/lib/_collection_dev/list.dart

Index: sdk/lib/_collection_dev/list.dart
diff --git a/sdk/lib/_collection_dev/list.dart b/sdk/lib/_collection_dev/list.dart
index d7b2661fd4da14d9ac4b5f7230698ffdd638f168..0260290dc1dd423987a17037d675354d6c6f9024 100644
--- a/sdk/lib/_collection_dev/list.dart
+++ b/sdk/lib/_collection_dev/list.dart
@@ -97,6 +97,10 @@ abstract class ListBase<E> extends Collection<E> implements List<E> {
     return new MappedIterable(this, f);
   }
 
+  List mappedBy(f(E element)) {
+    return new MappedList(this, f);
+  }
+
   Iterable<E> take(int n) {
     return new SubListIterable(this, 0, n);
   }
@@ -282,6 +286,16 @@ abstract class UnmodifiableListBase<E> extends ListBase<E> {
   }
 }
 
+class MappedList<S, T> extends UnmodifiableListBase<T> {
+  final List<S> _list;
+  // TODO(ahe): Restore type when feature is implemented in dart2js
+  // checked mode. http://dartbug.com/7733
+  final /* _Transformation<S, T> */ _f;
+
+  MappedList(this._list, T this._f(S element));
+
+  T operator[](int index) => _f(_list[index]);
+  int get length => _list.length;
 }
 
 /** An empty fixed-length list. */