SDK: take advantage of field promotion.

Now that field promotion (language feature `inference-update-2`) has
been enabled, we can take advantage of it in the core SDK.

Fixes #52982.

Change-Id: I7144ec26a7b4428f758f4d876d0fdce54a4e30ee
Bug: #52982
CoreLibraryReviewExempt: VM-only change
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/314640
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Lasse Nielsen <lrn@google.com>
Reviewed-by: Slava Egorov <vegorov@google.com>

sdk/lib/_http/http_impl.dart

@@ -703,12 +703,10 @@ class _HttpClientResponse extends _HttpInboundMessageListInt
           .transform(gzip.decoder)
           .transform(const _ToUint8List());
     }
-    // TODO(#52982): Make use of field promotion of `_profileData`.
-    var profileData = _profileData;
-    if (profileData != null) {
+    if (_profileData != null) {
       // If _timeline is not set up, don't add unnecessary map() to the stream.
       stream = stream.map((data) {
-        profileData.appendResponseData(data);
+        _profileData.appendResponseData(data);
         return data;
       });
     }
@@ -1152,13 +1150,11 @@ abstract class _HttpOutboundMessage<T> extends _IOSinkImpl {
   }
 
   Future addStream(Stream<List<int>> s) {
-    // TODO(#52982): Make use of field promotion of `_profileData`.
-    var profileData = _profileData;
-    if (profileData == null) {
+    if (_profileData == null) {
       return super.addStream(s);
     }
     return super.addStream(s.map((data) {
-      profileData.appendRequestData(Uint8List.fromList(data));
+      _profileData.appendRequestData(Uint8List.fromList(data));
       return data;
     }));
   }

sdk/lib/developer/timeline.dart

@@ -246,9 +246,7 @@ final class TimelineTask {
         map[key] = arguments[key];
       }
     }
-    // TODO(#52982): Make use of field promotion of `_parent`.
-    var parent = _parent;
-    if (parent != null) map['parentId'] = parent._taskId.toRadixString(16);
+    if (_parent != null) map['parentId'] = _parent._taskId.toRadixString(16);
     if (_filterKey != null) map[_kFilterKey] = _filterKey;
     block._start(map);
   }