Clarify dart:ui dependencies in foundation library (#24868)

Author: Hixie

packages/flutter/lib/src/foundation/README.md

@@ -2,3 +2,12 @@ The rule for packages in this directory is that they can depend on
 nothing but core Dart packages. They can't depend on `dart:ui`, they
 can't depend on any `package:`, and they can't depend on anything
 outside this directory.
+
+Currently they do depend on dart:ui, but only for `VoidCallback` and
+`hashValues` (and maybe one day `hashList` and `lerpDouble`), which
+are all intended to be moved out of `dart:ui` and into `dart:core`.
+
+See also:
+
+ * https://github.com/dart-lang/sdk/issues/27791 (`VoidCallback`)
+ * https://github.com/dart-lang/sdk/issues/25217 (`hashValues`, `hashList`, and `lerpDouble`)

packages/flutter/lib/src/foundation/profile.dart

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:ui';
+import 'dart:ui' show VoidCallback;
 
 /// Whether we've been built in release mode.
 const bool _kReleaseMode = bool.fromEnvironment('dart.vm.product');