Revert "Remove deprecated mappedBy."

utils/pub/utils.dart

Index: utils/pub/utils.dart
diff --git a/utils/pub/utils.dart b/utils/pub/utils.dart
index f7589dae271978f328664f0bb0b77899e9b04475..a058e48e68674063fd8980b8979ed53e6246dd55 100644
--- a/utils/pub/utils.dart
+++ b/utils/pub/utils.dart
@@ -274,13 +274,13 @@ Stream wrapStream(Stream stream) {
 /// Like [Iterable.where], but allows [test] to return [Future]s and uses the
 /// results of those [Future]s as the test.
 Future<Iterable> futureWhere(Iterable iter, test(value)) {
-  return Future.wait(iter.map((e) {
+  return Future.wait(iter.mappedBy((e) {
     var result = test(e);
     if (result is! Future) result = new Future.immediate(result);
     return result.then((result) => new Pair(e, result));
   }))
       .then((pairs) => pairs.where((pair) => pair.last))
-      .then((pairs) => pairs.map((pair) => pair.first));
+      .then((pairs) => pairs.mappedBy((pair) => pair.first));
 }
 
 // TODO(nweiz): unify the following functions with the utility functions in