Make --trace-loading great again.

runtime/bin/vmservice/loader.dart

Index: runtime/bin/vmservice/loader.dart
diff --git a/runtime/bin/vmservice/loader.dart b/runtime/bin/vmservice/loader.dart
index 0d9246a40c52d9886adaaa99508e091945206877..4b4d575d5bd7616b88cbd37ed812f01cc6876be7 100644
--- a/runtime/bin/vmservice/loader.dart
+++ b/runtime/bin/vmservice/loader.dart
@@ -62,6 +62,8 @@ class FileRequest {
   FileRequest(this.sp, this.tag, this.uri, this.resolvedUri, this.libraryUrl);
 }
 
+bool _traceLoading = false;
+
 // State associated with the isolate that is used for loading.
 class IsolateLoaderState extends IsolateEmbedderData {
   IsolateLoaderState(this.isolateId);
@@ -101,8 +103,6 @@ class IsolateLoaderState extends IsolateEmbedderData {
   // The root script's uri.
   Uri _rootScript;
 
-  bool _traceLoading = false;
-
   // Packages are either resolved looking up in a map or resolved from within a
   // package root.
   bool get _packagesReady =>