Add mirror overrides to doctor output (#56928)

packages/flutter_tools/lib/src/base/user_messages.dart

diff --git a/packages/flutter_tools/lib/src/base/user_messages.dart b/packages/flutter_tools/lib/src/base/user_messages.dart
index a1a3163..a5dc91b 100644
--- a/packages/flutter_tools/lib/src/base/user_messages.dart
+++ b/packages/flutter_tools/lib/src/base/user_messages.dart
@@ -22,6 +22,8 @@
       'Framework revision $revision ($age), $date';
   String engineRevision(String revision) => 'Engine revision $revision';
   String dartRevision(String revision) => 'Dart version $revision';
+  String pubMirrorURL(String url) => 'Pub download mirror $url';
+  String flutterMirrorURL(String url) => 'Flutter download mirror $url';
   String get flutterBinariesDoNotRun =>
       'Downloaded executables cannot execute on host.\n'
       'See https://github.com/flutter/flutter/issues/6207 for more information';

packages/flutter_tools/lib/src/doctor.dart

diff --git a/packages/flutter_tools/lib/src/doctor.dart b/packages/flutter_tools/lib/src/doctor.dart
index bad0a9d..c883bab 100644
--- a/packages/flutter_tools/lib/src/doctor.dart
+++ b/packages/flutter_tools/lib/src/doctor.dart
@@ -642,6 +642,12 @@
       )));
       messages.add(ValidationMessage(userMessages.engineRevision(version.engineRevisionShort)));
       messages.add(ValidationMessage(userMessages.dartRevision(version.dartSdkVersion)));
+      if (globals.platform.environment.containsKey('PUB_HOSTED_URL')) {
+        messages.add(ValidationMessage(userMessages.pubMirrorURL(globals.platform.environment['PUB_HOSTED_URL'])));
+      }
+      if (globals.platform.environment.containsKey('FLUTTER_STORAGE_BASE_URL')) {
+        messages.add(ValidationMessage(userMessages.flutterMirrorURL(globals.platform.environment['FLUTTER_STORAGE_BASE_URL'])));
+      }
     } on VersionCheckError catch (e) {
       messages.add(ValidationMessage.error(e.message));
       valid = ValidationType.partial;