feat: add keyName to asset generator (#264)

Author: wasabeef

example/lib/gen/assets.gen.dart

@@ -192,6 +192,8 @@ class AssetGenImage {
   }
 
   String get path => _assetName;
+
+  String get keyName => _assetName;
 }
 
 class SvgGenImage {

example_resources/lib/gen/assets.gen.dart

@@ -354,7 +354,12 @@ class $className {
 }
 
 String _assetGenImageClassDefinition(String packageName) {
-  final packageParameter = packageName.isNotEmpty ? ' = \'$packageName\'' : '';
+  final packageParameter = packageName.isNotEmpty ? " = '$packageName'" : '';
+
+  final keyName = packageName.isEmpty
+      ? '_assetName'
+      : "'packages/$packageName/\$_assetName'";
+
   return '''
 
 class AssetGenImage {
@@ -416,6 +421,8 @@ class AssetGenImage {
   }
 
   String get path => _assetName;
+
+  String get keyName => $keyName;
 }
 ''';
 }