Arrow syntax for getters (#156483)

This egotistical PR aims to draw attention to a [style guideline](https://github.com/flutter/flutter/blob/master/docs/contributing/Style-guide-for-Flutter-repo.md#use--for-getters-and-callbacks-that-just-return-literals-or-switch-expressions) that I changed:

> #### Use `=>` for getters and callbacks that just return literals or switch expressions

<br>

There was an opportunity for valuable discussion in #154753 about how this structure affects readability, but I shut it down pretty quick since there was a lot of other stuff going on there.

Interested to hear thoughts from @Piinks and others.

dev/devicelab/lib/framework/ab.dart

@@ -207,12 +207,10 @@ class ABTest {
     return buffer.toString();
   }
 
-  Set<String> get _allScoreKeys {
-    return <String>{
-      ..._aResults.keys,
-      ..._bResults.keys,
-    };
-  }
+  Set<String> get _allScoreKeys => <String>{
+    ..._aResults.keys,
+    ..._bResults.keys,
+  };
 
   /// Returns the summary as a tab-separated spreadsheet.
   ///