Reestablish abstract on HttpProfiler.

Class used to be abstract, but lost it when made `final`.
There is no reason to allow instantiation, it's all static members.
Restablishing the `abstract` before we release 3.0, so we don't have to go through any extra steps.

The class is public, exported by `dart:io`. (Should probably also be documented.)

Change-Id: Ic1a3e1a777ecbdcf535b92dbcdd88f55a65e9cbe
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/297463
Commit-Queue: Lasse Nielsen <[email protected]>
Reviewed-by: Slava Egorov <[email protected]>

Author: lrhn

sdk/lib/_http/http_impl.dart

@@ -4,7 +4,7 @@
 
 part of dart._http;
 
-final class HttpProfiler {
+abstract final class HttpProfiler {
   static const _kType = 'HttpProfile';
 
   static final Map<String, _HttpProfileData> _profile = {};