Sort procedures by source positions.

pkg/front_end/test/fasta/accessors.dart.outline.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class C extends core::Object {
   constructor •() → void
     ;
   set onlySetter(dynamic value) → void
     ;
   method testC() → dynamic
     ;
   method testD() → dynamic
     ;
 }
 class D extends self::C {
   constructor •() → void
     ;
+  get onlySetter() → core::String
+    ;
   set onlySetter(dynamic value) → void
     ;
-  get onlySetter() → core::String
-    ;
 }
 static set onlySetter(dynamic value) → void
   ;
 static method main() → dynamic
   ;