Make 'where' 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 e5d994356a8ce0368341d5bb7440d15e45d5e3a8..6b3adb16673dce6d2efd2fc534a7ba39e7990f3f 100644
--- a/utils/tests/pub/test_pub.dart
+++ b/utils/tests/pub/test_pub.dart
@@ -783,8 +783,8 @@ abstract class Descriptor {
     }
 
     return listDir(dir).chain((files) {
-      var matches = files.where((file) => endsWithPattern(file, name));
-      if (matches.length == 0) {
+      var matches = files.where((file) => endsWithPattern(file, name)).toList();
+      if (matches.isEmpty) {
         Expect.fail('No files in $dir match pattern $name.');
       }
       if (matches.length == 1) return validate(matches[0]);