only build asset when there is asset declared in pubspec (#31804)

packages/flutter_test/lib/src/binding.dart

@@ -740,7 +740,8 @@ class AutomatedTestWidgetsFlutterBinding extends TestWidgetsFlutterBinding {
   @override
   int get microtaskCount => _currentFakeAsync.microtaskCount;
 
-  static Set<String> _allowedKeys;
+  /// A whitelist [Set] that is used in mocking the asset message channel.
+  static Set<String> _allowedAssetKeys;
 
   void _mockFlutterAssets() {
     if (!Platform.environment.containsKey('UNIT_TEST_ASSETS')) {
@@ -748,29 +749,39 @@ class AutomatedTestWidgetsFlutterBinding extends TestWidgetsFlutterBinding {
     }
     final String assetFolderPath = Platform.environment['UNIT_TEST_ASSETS'];
     _ensureInitialized(assetFolderPath);
+
+    if (_allowedAssetKeys.isNotEmpty) {
       BinaryMessages.setMockMessageHandler('flutter/assets', (ByteData message) {
         final String key = utf8.decode(message.buffer.asUint8List());
-      if (_allowedKeys.contains(key)) {
+        if (_allowedAssetKeys.contains(key)) {
           final File asset = File(path.join(assetFolderPath, key));
           final Uint8List encoded = Uint8List.fromList(asset.readAsBytesSync());
           return Future<ByteData>.value(encoded.buffer.asByteData());
         }
       });
     }
+  }
 
   void _ensureInitialized(String assetFolderPath) {
-    if (_allowedKeys == null) {
+    if (_allowedAssetKeys != null) {
+      return;
+    }
     final File manifestFile = File(
         path.join(assetFolderPath, 'AssetManifest.json'));
+    // If the file does not exist, it means there is no asset declared in
+    // the project.
+    if (!manifestFile.existsSync()) {
+      _allowedAssetKeys = <String>{};
+      return;
+    }
     final Map<String, dynamic> manifest = json.decode(
         manifestFile.readAsStringSync());
-      _allowedKeys = <String>{
+    _allowedAssetKeys = <String>{
       'AssetManifest.json',
     };
     for (List<dynamic> value in manifest.values) {
       final List<String> strList = List<String>.from(value);
-        _allowedKeys.addAll(strList);
-      }
+      _allowedAssetKeys.addAll(strList);
     }
   }
 

packages/flutter_tools/lib/src/commands/test.dart

@@ -127,13 +127,14 @@ class TestCommand extends FastFlutterCommand {
       await pubGet(context: PubContext.getVerifyContext(name), skipPubspecYamlCheck: true);
     }
     final bool buildTestAssets = argResults['test-assets'];
-    if (buildTestAssets) {
-      await _buildTestAsset();
-    }
     final List<String> names = argResults['name'];
     final List<String> plainNames = argResults['plain-name'];
     final FlutterProject flutterProject = FlutterProject.current();
 
+    if (buildTestAssets && flutterProject.manifest.assets.isNotEmpty) {
+      await _buildTestAsset();
+    }
+
     Iterable<String> files = argResults.rest.map<String>((String testPath) => fs.path.absolute(testPath)).toList();
 
     final bool startPaused = argResults['start-paused'];