Mark kernel test as slow, regenerate baseline file

pkg/kernel/testcases/strong-mode/covariant_generic.baseline.txt

 library;
 import self as self;
 import "dart:core" as core;
 
 class Foo<T extends core::Object> extends core::Object {
   final field self::Foo::T finalField;
   final field (self::Foo::T) → void callbackField;
   field self::Foo::T mutableField = null;
   field (self::Foo::T) → void mutableCallbackField = null;
   constructor •(self::Foo::T finalField, (self::Foo::T) → void callbackField) → void
     : self::Foo::finalField = finalField, self::Foo::callbackField = callbackField, super core::Object::•()
     ;
   method method(self::Foo::T x) → void {}
   set setter(self::Foo::T x) → void {}
   method withCallback((self::Foo::T) → void callback) → void {
     callback.call(this.{self::Foo::finalField});
   }
   set /* from null */ mutableField$cc(core::Object mutableField_) → dynamic
     this.{=self::Foo::mutableField} = mutableField_ as self::Foo::T;
   method method$cc(core::Object x) → void
     this.{=self::Foo::method}(x as self::Foo::T);
   set setter$cc(core::Object x) → void
     this.{=self::Foo::setter} = x as self::Foo::T;
 }
 static method main() → dynamic {
-  self::Foo<core::int> fooInt = new self::Foo::•<core::int>(1, (core::int x) → void {});
+  self::Foo<core::int> fooInt = new self::Foo::•<core::int>(1, (core::int x) → core::Null {});
   fooInt.{self::Foo::method}(3);
   fooInt.{self::Foo::setter} = 3;
-  fooInt.{self::Foo::withCallback}((core::int x) → void {});
-  fooInt.{self::Foo::withCallback}((core::num x) → void {});
+  fooInt.{self::Foo::withCallback}((core::int x) → core::Null {});
+  fooInt.{self::Foo::withCallback}((core::num x) → core::Null {});
   fooInt.{self::Foo::mutableField} = 3;
-  fooInt.{self::Foo::mutableCallbackField} = (core::int x) → void {};
+  fooInt.{self::Foo::mutableCallbackField} = (core::int x) → core::Null {};
   self::Foo<core::num> fooNum = fooInt;
   fooNum.{self::Foo::method$cc}(3);
   fooNum.{self::Foo::method$cc}(2.5);
   fooNum.{self::Foo::setter$cc} = 3;
   fooNum.{self::Foo::setter$cc} = 2.5;
-  fooNum.{self::Foo::withCallback}((core::num x) → void {});
+  fooNum.{self::Foo::withCallback}((core::num x) → core::Null {});
   fooNum.{self::Foo::mutableField$cc} = 3;
   fooNum.{self::Foo::mutableField$cc} = 2.5;
   fooNum.{self::Foo::mutableCallbackField}.call(3);
   fooNum.{self::Foo::mutableCallbackField}.call(2.5);
-  fooNum.{self::Foo::mutableCallbackField} = (core::num x) → void {};
+  fooNum.{self::Foo::mutableCallbackField} = (core::num x) → core::Null {};
 }