[flutter_tool] add a vmservice API for hot ui requests (#45649)

diff --git a/packages/flutter_tools/lib/src/devfs.dart b/packages/flutter_tools/lib/src/devfs.dart
index 1c076d4..1e329e8 100644
--- a/packages/flutter_tools/lib/src/devfs.dart
+++ b/packages/flutter_tools/lib/src/devfs.dart
@@ -453,6 +453,7 @@
     String projectRootPath,
     @required String pathToReload,
     @required List<Uri> invalidatedFiles,
+    bool skipAssets = false,
   }) async {
     assert(trackWidgetCreation != null);
     assert(generator != null);
@@ -463,7 +464,7 @@
     final Map<Uri, DevFSContent> dirtyEntries = <Uri, DevFSContent>{};
 
     int syncedBytes = 0;
-    if (bundle != null) {
+    if (bundle != null && !skipAssets) {
       printTrace('Scanning asset files');
       // We write the assets into the AssetBundle working dir so that they
       // are in the same location in DevFS and the iOS simulator.