Don't print stacktraces from ToolExit errors during flutter run. (#8453)

Instead, actually exit the tool.

Fixes #8363.

packages/flutter_tools/lib/src/run_cold.dart

@@ -8,6 +8,7 @@ import 'package:meta/meta.dart';
 import 'package:stack_trace/stack_trace.dart';
 
 import 'application_package.dart';
+import 'base/common.dart';
 import 'base/file_system.dart';
 import 'base/utils.dart';
 import 'build_info.dart';
@@ -54,6 +55,9 @@ class ColdRunner extends ResidentRunner {
         shouldBuild: shouldBuild
       );
     }, onError: (dynamic error, StackTrace stackTrace) {
+      // Actually exit on ToolExit.
+      if (error is ToolExit)
+        throw error;
       printError('Exception from flutter run: $error', stackTrace);
     });
   }

packages/flutter_tools/lib/src/run_hot.dart

@@ -8,6 +8,7 @@ import 'package:meta/meta.dart';
 import 'package:stack_trace/stack_trace.dart';
 
 import 'application_package.dart';
+import 'base/common.dart';
 import 'base/context.dart';
 import 'base/file_system.dart';
 import 'base/logger.dart';
@@ -81,6 +82,9 @@ class HotRunner extends ResidentRunner {
         shouldBuild: shouldBuild
       );
     }, onError: (dynamic error, StackTrace stackTrace) {
+      // Actually exit on ToolExit.
+      if (error is ToolExit)
+        throw error;
       printError('Exception from flutter run: $error', stackTrace);
     });
   }