Fix expected error output.

sdk/lib/_internal/pub/test/transformer/fails_to_load_a_transform_with_an_import_error_test.dart

Index: sdk/lib/_internal/pub/test/transformer/fails_to_load_a_transform_with_an_import_error_test.dart
===================================================================
--- sdk/lib/_internal/pub/test/transformer/fails_to_load_a_transform_with_an_import_error_test.dart	(revision 30305)
+++ sdk/lib/_internal/pub/test/transformer/fails_to_load_a_transform_with_an_import_error_test.dart	(working copy)
@@ -29,7 +29,7 @@
     createLockFile('myapp', pkg: ['barback']);
     var pub = startPubServe();
     expect(pub.nextErrLine(), completion(matches(new RegExp(
-        r"Error: line 1 pos 1: library handler failed$"))));
+        r"error: line 1 pos 1: library handler failed$"))));
     pub.shouldExit(1);
   });
 }