Revert "Remove deprecated mappedBy."

sdk/lib/_internal/compiler/implementation/lib/js_array.dart

Index: sdk/lib/_internal/compiler/implementation/lib/js_array.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/js_array.dart b/sdk/lib/_internal/compiler/implementation/lib/js_array.dart
index 1c1e4920dbf0511057e3a10a6c2f10b06d87cfc8..91a6782fd96f3eaf8d5eb48e7fc76b80ae8595f8 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/js_array.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/js_array.dart
@@ -92,6 +92,10 @@ class JSArray<E> implements List<E> {
     return IterableMixinWorkaround.mapList(this, f);
   }
 
+  List mappedBy(f(E element)) {
+    return IterableMixinWorkaround.mappedByList(this, f);
+  }
+
   String join([String separator]) {
     if (separator == null) separator = "";
     var list = new List(this.length);