Another attempt to fix the windows tests

pkg/analysis_server/test/abstract_context.dart

Index: pkg/analysis_server/test/abstract_context.dart
diff --git a/pkg/analysis_server/test/abstract_context.dart b/pkg/analysis_server/test/abstract_context.dart
index 4e3eb07e9db121013ee5935436e4fa7ac1e0980e..c5fb943c7330344598e1a641f52a4e810dbc7e01 100644
--- a/pkg/analysis_server/test/abstract_context.dart
+++ b/pkg/analysis_server/test/abstract_context.dart
@@ -98,6 +98,9 @@ class Required {
   }
 
   Source addSource(String path, String content, [Uri uri]) {
+    if (path.startsWith('/')) {
+      path = provider.convertPath(path);
+    }
     File file = newFile(path, content);
     Source source = file.createSource(uri);
     if (enableNewAnalysisDriver) {