make sure we init flutter_tools with a fallback locale (#17877)

packages/flutter_tools/lib/runner.dart

@@ -5,7 +5,8 @@
 import 'dart:async';
 
 import 'package:args/command_runner.dart';
-import 'package:intl/intl_standalone.dart' as intl;
+import 'package:intl/intl.dart' as intl;
+import 'package:intl/intl_standalone.dart' as intl_standalone;
 import 'package:meta/meta.dart';
 
 import 'src/base/common.dart';
@@ -48,7 +49,11 @@ Future<int> run(
 
   return runInContext<int>(() async {
     // Initialize the system locale.
-    await intl.findSystemLocale();
+    final String systemLocale = await intl_standalone.findSystemLocale();
+    intl.Intl.defaultLocale = intl.Intl.verifiedLocale(
+      systemLocale, intl.NumberFormat.localeExists,
+      onFailure: (String _) => 'en_US'
+    );
 
     try {
       await runner.run(args);