Test again the library url in the error message shortened in r3965.

runtime/vm/dart_api_impl_test.cc

Index: runtime/vm/dart_api_impl_test.cc
===================================================================
--- runtime/vm/dart_api_impl_test.cc	(revision 3965)
+++ runtime/vm/dart_api_impl_test.cc	(working copy)
@@ -1495,7 +1495,10 @@
   // We expect the test script to fail finalization with the error below:
   EXPECT(Dart_IsError(result));
   Dart_Handle expected_error = Dart_Error(
-      "'dart:test-lib': Error: line 1 pos 38: class 'NativeFields' is trying");
+      "'dart:test-lib': Error: line 1 pos 38: "
+      "class 'NativeFields' is trying to extend a native fields class, "
+      "but library '%s' has no native resolvers",
+      TestCase::url());
   EXPECT_SUBSTRING(Dart_GetError(expected_error), Dart_GetError(result));
 }