Move kernel baseline tests to front_end.

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

 library;
 import self as self;
 import "dart:core" as core;
 
 static method foo(core::int x, core::int y) → core::int {
   core::int z = x.{core::num::+}(y);
   return z.{core::int::<<}(4);
 }
 static method loop(core::List<dynamic> xs) → void {
   core::int _ = xs.{core::List::length};
   for (core::int i = 0; i.{core::num::<}(xs.{core::List::length}); i = i.{core::num::+}(1)) {
   }
 }
 static method main() → dynamic {
   self::foo(4, 5);
   self::foo(6, 7);
   self::loop(<dynamic>["dfg"]);
 }