Make mappedBy lazy.

utils/tests/pub/test_pub.dart

Index: utils/tests/pub/test_pub.dart
diff --git a/utils/tests/pub/test_pub.dart b/utils/tests/pub/test_pub.dart
index 8ffabbc66d0ef22c857629661c035cc441750404..e5d994356a8ce0368341d5bb7440d15e45d5e3a8 100644
--- a/utils/tests/pub/test_pub.dart
+++ b/utils/tests/pub/test_pub.dart
@@ -910,7 +910,8 @@ class DirectoryDescriptor extends Descriptor {
       if (contents == null) return new Future<Directory>.immediate(dir);
 
       // Recursively create all of its children.
-      final childFutures = contents.mappedBy((child) => child.create(dir));
+      final childFutures =
+          contents.mappedBy((child) => child.create(dir)).toList();
       // Only complete once all of the children have been created too.
       return Futures.wait(childFutures).transform((_) => dir);
     });
@@ -933,7 +934,8 @@ class DirectoryDescriptor extends Descriptor {
   Future validate(String path) {
     return _validateOneMatch(path, (dir) {
       // Validate each of the items in this directory.
-      final entryFutures = contents.mappedBy((entry) => entry.validate(dir));
+      final entryFutures =
+          contents.mappedBy((entry) => entry.validate(dir)).toList();
 
       // If they are all valid, the directory is valid.
       return Futures.wait(entryFutures).transform((entries) => null);
@@ -1198,7 +1200,9 @@ class NothingDescriptor extends Descriptor {
 Future _awaitObject(object) {
   // Unroll nested futures.
   if (object is Future) return object.chain(_awaitObject);
-  if (object is Collection) return Futures.wait(object.mappedBy(_awaitObject));
+  if (object is Collection) {
+    return Futures.wait(object.mappedBy(_awaitObject).toList());
+  }
   if (object is! Map) return new Future.immediate(object);
 
   var pairs = <Future<Pair>>[];