Update some http links to https (#14762)

These URLs all automatically redirect HTTP to HTTPS so makes sense to point users at the secure versions.

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

@@ -206,7 +206,7 @@ class IOSWorkflow extends DoctorValidator implements Workflow {
       brewStatus = ValidationType.missing;
       messages.add(new ValidationMessage.error(
         'Brew not installed; use this to install tools for iOS device development.\n'
-        'Download brew at http://brew.sh/.'
+        'Download brew at https://brew.sh/.'
       ));
     }
 

packages/flutter_tools/pubspec.yaml

@@ -1,6 +1,6 @@
 name: flutter_tools
 description: Tools for building Flutter applications
-homepage: http://flutter.io
+homepage: https://flutter.io
 author: Flutter Authors <flutter-dev@googlegroups.com>
 
 environment:

packages/flutter_tools/templates/create/README.md.tmpl

@@ -5,4 +5,4 @@
 ## Getting Started
 
 For help getting started with Flutter, view our online
-[documentation](http://flutter.io/).
+[documentation](https://flutter.io/).

packages/flutter_tools/templates/package/README.md.tmpl

@@ -4,6 +4,6 @@
 
 ## Getting Started
 
-For help getting started with Flutter, view our online [documentation](http://flutter.io/).
+For help getting started with Flutter, view our online [documentation](https://flutter.io/).
 
 For help on editing package code, view the [documentation](https://flutter.io/developing-packages/).

packages/flutter_tools/templates/plugin/README.md.tmpl

@@ -5,6 +5,6 @@
 ## Getting Started
 
 For help getting started with Flutter, view our online
-[documentation](http://flutter.io/).
+[documentation](https://flutter.io/).
 
 For help on editing plugin code, view the [documentation](https://flutter.io/platform-plugins/#edit-code).