Pass a URI to package:http APIs (#2540)

Prepare for dart-lang/http#375

Author: natebosch

packages/devtools_app/lib/src/survey.dart

@@ -17,8 +17,8 @@ class SurveyService {
 
   static const _maxShowSurveyCount = 5;
 
-  static const _metadataUrl =
-      'https://flutter.dev/f/dart-devtools-survey-metadata.json';
+  static final _metadataUrl =
+      Uri.https('flutter.dev', '/f/dart-devtools-survey-metadata.json');
 
   /// Duration for which we should show the survey notification.
   ///