Move kernel baseline tests to front_end.

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

-library;
-import self as self;
-import "dart:async" as asy;
-import "dart:core" as core;
-
-static field core::List<core::String> stringList = <dynamic>["bar"];
-static method asyncString() → asy::Future<core::String> async {
-  return "foo";
-}
-static method asyncString2() → asy::Future<core::String> async {
-  return self::asyncString();
-}
-static method syncStarString() → core::Iterable<core::String> sync* {
-  yield "foo";
-  yield* self::syncStarString2();
-  yield* self::stringList;
-}
-static method syncStarString2() → core::Iterable<core::String> sync* {
-  yield "foo";
-}
-static method asyncStarString() → asy::Stream<core::String> async* {
-  yield "foo";
-  yield* self::asyncStarString2();
-  yield await self::asyncString();
-}
-static method asyncStarString2() → asy::Stream<core::String> async* {
-  yield "bar";
-}
-static method main() → dynamic async {
-  core::String str = await self::asyncString();
-}