Minor tweaks to flutter doctor (#9963)

packages/flutter_tools/lib/src/android/android_workflow.dart

@@ -104,7 +104,7 @@ class AndroidWorkflow extends DoctorValidator implements Workflow {
       } else {
         messages.add(new ValidationMessage.error(
           'Unable to locate Android SDK.\n'
-          'Install Android Studio from https://developer.android.com/studio/index.html.\n'
+          'Install Android Studio from: https://developer.android.com/studio/index.html\n'
           'On first launch it will assist you in installing the Android SDK components.\n'
           '(or visit https://flutter.io/setup/#android-setup for detailed instructions).\n'
           'If Android SDK has been installed to a custom location, set \$$kAndroidHome to that location.'

packages/flutter_tools/lib/src/android/gradle.dart

@@ -133,9 +133,8 @@ String ensureGradle() {
   String gradle = locateProjectGradlew();
   if (gradle == null) {
     gradle = locateSystemGradle();
-    if (gradle == null) {
+    if (gradle == null)
       throwToolExit('Unable to locate gradle. Please install Android Studio.');
-    }
   }
   printTrace('Using gradle from $gradle.');
   return gradle;

packages/flutter_tools/lib/src/commands/doctor.dart

@@ -11,6 +11,7 @@ class DoctorCommand extends FlutterCommand {
   DoctorCommand() {
     argParser.addFlag('android-licenses',
       defaultsTo: false,
+      negatable: false,
       help: 'Run the Android SDK manager tool to accept the SDK\'s licenses.',
     );
   }