Revert "Remove deprecated mappedBy."

sdk/lib/_collection_dev/iterable.dart

Index: sdk/lib/_collection_dev/iterable.dart
diff --git a/sdk/lib/_collection_dev/iterable.dart b/sdk/lib/_collection_dev/iterable.dart
index 676eadec18a38a84b14ed54efb7c10794aa44304..5df737881d45471f4a08799043fb9f0842a44714 100644
--- a/sdk/lib/_collection_dev/iterable.dart
+++ b/sdk/lib/_collection_dev/iterable.dart
@@ -197,6 +197,8 @@ abstract class ListIterable<E> extends Iterable<E> {
 
   Iterable map(f(E element)) => new MappedIterable(this, f);
 
+  Iterable mappedBy(f(E element)) => super.mappedBy(f);
+
   reduce(var initialValue, combine(var previousValue, E element)) {
     var value = initialValue;
     int length = this.length;
@@ -661,6 +663,8 @@ class EmptyIterable<E> extends Iterable<E> {
 
   Iterable map(f(E element)) => const EmptyIterable();
 
+  Iterable mappedBy(f(E element)) => const EmptyIterable();
+
   reduce(var initialValue, combine(var previousValue, E element)) {
     return initialValue;
   }