Add missing copyright on test cases.

pkg/front_end/testcases/void_methods.dart.strong.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class Foo extends core::Object {
   field core::List<dynamic> list = <dynamic>[1, 2, 3];
   default constructor •() → void
     : super core::Object::•()
     ;
   set first(dynamic x) → void
     this.list.[]=(0, x);
   operator []=(dynamic x, dynamic y) → void
     this.list.[]=(x, y);
   method clear() → void
     return this.{self::Foo::list}.{core::List::clear}();
 }
 static method main() → dynamic {
   new self::Foo::•().{self::Foo::first} = 4;
   new self::Foo::•().{self::Foo::[]=}(3, 4);
   new self::Foo::•().{self::Foo::clear}();
 }