[dart2wasm] Allow package:ui/* to import dart:_wasm

Change-Id: I4fc17b992db272d6154a7c58fc1f4229c5368ee4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/325620
Reviewed-by: Jackson Gardner <jacksongardner@google.com>
Auto-Submit: Martin Kustermann <kustermann@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>
Commit-Queue: Jackson Gardner <jacksongardner@google.com>

pkg/dart2wasm/lib/target.dart

@@ -186,7 +186,10 @@ class WasmTarget extends Target {
   bool allowPlatformPrivateLibraryAccess(Uri importer, Uri imported) =>
       super.allowPlatformPrivateLibraryAccess(importer, imported) ||
       importer.path.contains('tests/web/wasm') ||
-      importer.isScheme('package') && importer.path == 'js/js.dart';
+      importer.isScheme('package') &&
+          (importer.path == 'js/js.dart' ||
+              importer.path.startsWith('ui/') &&
+                  imported.toString() == 'dart:_wasm');
 
   void _patchHostEndian(CoreTypes coreTypes) {
     // Fix Endian.host to be a const field equal to Endian.little instead of