Move kernel baseline tests to front_end.

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

-library;
-import self as self;
-import "dart:core" as core;
-
-class Uninitialized extends core::Object {
-  field core::int x = null;
-  constructor •() → void
-    : super core::Object::•()
-    ;
-}
-class PartiallyInitialized extends core::Object {
-  field core::int x = null;
-  constructor •(core::int x) → void
-    : self::PartiallyInitialized::x = x, super core::Object::•()
-    ;
-  constructor noInitializer() → void
-    : super core::Object::•()
-    ;
-}
-class Initialized extends core::Object {
-  field core::int x;
-  constructor •(core::int x) → void
-    : self::Initialized::x = x, super core::Object::•()
-    ;
-}
-class Forwarding extends core::Object {
-  field core::int x;
-  constructor initialize(core::int x) → void
-    : self::Forwarding::x = x, super core::Object::•()
-    ;
-  constructor •(core::int arg) → void
-    : this self::Forwarding::initialize(arg)
-    ;
-}
-static field core::int uninitializedTopLevel = null;
-static field core::int initializedTopLevel = 4;
-static method main() → dynamic {}