Move kernel baseline tests to front_end.

pkg/kernel/testcases/spec-mode/arithmetic.baseline.txt

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