ensure lastBuildTimestamp is set before early return (#25058)

packages/flutter_tools/lib/src/asset.dart

@@ -107,6 +107,10 @@ class _ManifestAssetBundle implements AssetBundle {
     if (flutterManifest == null)
       return 1;
 
+    // If the last build time isn't set before this early return, empty pubspecs will
+    // hang on hot reload, as the incremental dill files will never be copied to the
+    // device.
+    _lastBuildTimestamp = DateTime.now();
     if (flutterManifest.isEmpty) {
       entries[_assetManifestJson] = DevFSStringContent('{}');
       return 0;
@@ -114,8 +118,6 @@ class _ManifestAssetBundle implements AssetBundle {
 
     final String assetBasePath = fs.path.dirname(fs.path.absolute(manifestPath));
 
-    _lastBuildTimestamp = DateTime.now();
-
     final PackageMap packageMap = PackageMap(packagesPath);
 
     // The _assetVariants map contains an entry for each asset listed

packages/flutter_tools/test/asset_test.dart

@@ -41,8 +41,26 @@ void main() {
         await getValueAsString('FontManifest.json', asset),
         '[{"family":"packages/font/test_font","fonts":[{"asset":"packages/font/test_font_file"}]}]',
       );
+      expect(asset.wasBuiltOnce(), true);
     });
 
+    testUsingContext('handles empty pubspec with .packages', () async {
+      final String dataPath = fs.path.join(
+        getFlutterRoot(),
+        'packages',
+        'flutter_tools',
+        'test',
+        'data',
+        'fuchsia_test',
+      );
+      final AssetBundle asset = AssetBundleFactory.instance.createBundle();
+      await asset.build(
+        manifestPath : fs.path.join(dataPath, 'main', 'pubspec.yaml'), // file doesn't exist
+        packagesPath: fs.path.join(dataPath, 'main', '.packages'),
+        includeDefaultFonts: false,
+      );
+      expect(asset.wasBuiltOnce(), true);
+    });
   });
 }
 

packages/flutter_tools/test/data/fuchsia_test/main/.gitignore

@@ -0,0 +1 @@
+!.packages

packages/flutter_tools/test/data/fuchsia_test/main/.packages

@@ -0,0 +1 @@
+testo:lib/