[ Service ] Fix `assets` getter being non-nullable when null is a valid
result

Fixes https://github.com/flutter/flutter/issues/101242

Change-Id: I13f24b96a36af607aa0273f17177722a730edc2a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/240120
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Ben Konyi <bkonyi@google.com>

sdk/lib/_internal/vm/bin/vmservice_server.dart

diff --git a/sdk/lib/_internal/vm/bin/vmservice_server.dart b/sdk/lib/_internal/vm/bin/vmservice_server.dart
index 03ea982..2fdff43 100644
--- a/sdk/lib/_internal/vm/bin/vmservice_server.dart
+++ b/sdk/lib/_internal/vm/bin/vmservice_server.dart
@@ -373,7 +373,7 @@
       request.response.close();
       return;
     }
-    final asset = assets[path];
+    final asset = assets![path];
     if (asset != null) {
       // Serving up a static asset (e.g. .css, .html, .png).
       request.response.headers.contentType = ContentType.parse(asset.mimeType);

sdk/lib/vmservice/asset.dart

diff --git a/sdk/lib/vmservice/asset.dart b/sdk/lib/vmservice/asset.dart
index 985c905..7e086ff 100644
--- a/sdk/lib/vmservice/asset.dart
+++ b/sdk/lib/vmservice/asset.dart
@@ -58,7 +58,7 @@
 external List<Object> _decodeAssets(Uint8List data);
 
 Map<String, Asset>? _assets;
-Map<String, Asset> get assets {
+Map<String, Asset>? get assets {
   if (_assets == null) {
     try {
       _assets = Asset.request();
@@ -66,5 +66,5 @@
       print('Could not load Observatory assets: $e');
     }
   }
-  return _assets!;
+  return _assets;
 }