Revert "Remove deprecated mappedBy."

runtime/lib/array.dart

Index: runtime/lib/array.dart
diff --git a/runtime/lib/array.dart b/runtime/lib/array.dart
index 1441dc2a50e4b34a97de693994693f7f365f85ea..0f02eb4c01c30ff062241f0ecba9190cf985d55f 100644
--- a/runtime/lib/array.dart
+++ b/runtime/lib/array.dart
@@ -104,6 +104,10 @@ class _ObjectArray<E> implements List<E> {
     return IterableMixinWorkaround.mapList(this, f);
   }
 
+  List mappedBy(f(E element)) {
+    IterableMixinWorkaround.mappedByList(this, f);
+  }
+
   reduce(initialValue, combine(previousValue, E element)) {
     return IterableMixinWorkaround.reduce(this, initialValue, combine);
   }
@@ -334,6 +338,10 @@ class _ImmutableArray<E> implements List<E> {
     return IterableMixinWorkaround.mapList(this, f);
   }
 
+  List mappedBy(f(E element)) {
+    return IterableMixinWorkaround.mappedByList(this, f);
+  }
+
   String join([String separator]) {
     return IterableMixinWorkaround.joinList(this, separator);
   }