some whitespace cleanup (#14443)

diff --git a/packages/flutter_tools/test/ios/mac_test.dart b/packages/flutter_tools/test/ios/mac_test.dart
index 7b59f5e..987c737 100644
--- a/packages/flutter_tools/test/ios/mac_test.dart
+++ b/packages/flutter_tools/test/ios/mac_test.dart
@@ -81,7 +81,7 @@
 
       testUsingContext('idevicescreenshot captures and returns screenshot', () async {
         when(mockOutputFile.path).thenReturn(outputPath);
-        when(mockProcessManager.run(any, environment: null, workingDirectory:  null)).thenAnswer(
+        when(mockProcessManager.run(any, environment: null, workingDirectory: null)).thenAnswer(
             (Invocation invocation) => new Future<ProcessResult>.value(new ProcessResult(4, 0, '', '')));
 
         await iMobileDevice.takeScreenshot(mockOutputFile);