Revert "Remove deprecated mappedBy."

runtime/lib/growable_array.dart

Index: runtime/lib/growable_array.dart
diff --git a/runtime/lib/growable_array.dart b/runtime/lib/growable_array.dart
index 27699e2efe384a2f6f9bee51e1844c91b6acabbc..d030e64acc1342ec1559fe33186c6a82f510d43c 100644
--- a/runtime/lib/growable_array.dart
+++ b/runtime/lib/growable_array.dart
@@ -246,6 +246,10 @@ class _GrowableObjectArray<T> implements List<T> {
     return IterableMixinWorkaround.mapList(this, f);
   }
 
+  List mappedBy(f(T element)) {
+    return IterableMixinWorkaround.mappedByList(this, f);
+  }
+
   reduce(initialValue, combine(previousValue, T element)) {
     return IterableMixinWorkaround.reduce(this, initialValue, combine);
   }