brew update is not needed any more: Homebrew/brew#679 (#11406)

packages/flutter_tools/lib/src/ios/cocoapods.dart

@@ -23,12 +23,10 @@ final String noCocoaPodsConsequence = '''
   For more info, see https://flutter.io/platform-plugins''';
 
 final String cocoaPodsInstallInstructions = '''
-  brew update
   brew install cocoapods
   pod setup''';
 
 final String cocoaPodsUpgradeInstructions = '''
-  brew update
   brew upgrade cocoapods
   pod setup''';
 

packages/flutter_tools/lib/src/ios/devices.dart

@@ -21,7 +21,6 @@ import 'mac.dart';
 
 const String _kIdeviceinstallerInstructions =
     'To work with iOS devices, please install ideviceinstaller. To install, run:\n'
-    'brew update\n'
     'brew install ideviceinstaller.';
 
 const Duration kPortForwardTimeout = const Duration(seconds: 10);
@@ -46,7 +45,6 @@ class IOSDevice extends Device {
     _pusherPath = _checkForCommand(
       'ios-deploy',
       'To copy files to iOS devices, please install ios-deploy. To install, run:\n'
-      'brew update\n'
       'brew install ios-deploy'
     );
   }

packages/flutter_tools/lib/src/ios/ios_workflow.dart

@@ -129,7 +129,6 @@ class IOSWorkflow extends DoctorValidator implements Workflow {
         brewStatus = ValidationType.partial;
         messages.add(new ValidationMessage.error(
             'libimobiledevice and ideviceinstaller are not installed or require updating. To update, run:\n'
-            '  brew update\n'
             '  brew uninstall --ignore-dependencies libimobiledevice\n'
             '  brew install --HEAD libimobiledevice\n'
             '  brew install ideviceinstaller'
@@ -139,7 +138,6 @@ class IOSWorkflow extends DoctorValidator implements Workflow {
         messages.add(new ValidationMessage.error(
           'ideviceinstaller not available; this is used to discover connected iOS devices.\n'
           'To install, run:\n'
-          '  brew update\n'
           '  brew install --HEAD libimobiledevice\n'
           '  brew install ideviceinstaller'
         ));
@@ -154,13 +152,11 @@ class IOSWorkflow extends DoctorValidator implements Workflow {
         if (await hasIosDeploy) {
           messages.add(new ValidationMessage.error(
             'ios-deploy out of date ($iosDeployMinimumVersion is required). To upgrade:\n'
-            '  brew update\n'
             '  brew upgrade ios-deploy'
           ));
         } else {
           messages.add(new ValidationMessage.error(
             'ios-deploy not installed. To install:\n'
-            '  brew update\n'
             '  brew install ios-deploy'
           ));
         }