1-在flutter包中的lib文件夹的同一级别创建assets文件夹，并将您的字体或图像

2-在pubspec.yaml中添加您的资产路径

3-创建widget时设置包名

我不确定flutter的两个集成项目是如何工作的，所以如果这是一个愚蠢的回应，我提前道歉，但你知道pubspec.yaml中的资产依赖行（最后两行）被注解掉了吗？

// you are not correctly defining the assets seen in your code under flutter uses-material-design: true assets folder should be like this 
        
         uses-material-design: true
         assets:
           - packages/project_a/images/
    
    if You do not use the proper format you will get an error, Thanks this will help you out. I am sharing my project pubsec.yml file check this 
          
        
        /////pubsec.yml file
        name: traveling
        description: A new Flutter application.
        
        # The following line prevents the package from being accidentally published to
        # pub.dev using `pub publish`. This is preferred for private packages.
        publish_to: 'none' # Remove this line if you wish to publish to pub.dev
        
        # The following defines the version and build number for your application.
        # A version number is three numbers separated by dots, like 1.2.43
        # followed by an optional build number separated by a +.
        # Both the version and the builder number may be overridden in flutter
        # build by specifying --build-name and --build-number, respectively.
        # In Android, build-name is used as versionName while build-number used as versionCode.
        # Read more about Android versioning at https://developer.android.com/studio/publish/versioning
        # In iOS, build-name is used as CFBundleShortVersionString while build-number used as CFBundleVersion.
        # Read more about iOS versioning at
        # https://developer.apple.com/library/archive/documentation/General/Reference/InfoPlistKeyReference/Articles/CoreFoundationKeys.html
        version: 1.0.0+1
        
        environment:
          sdk: ">=2.12.0 <3.0.0"
        
        dependencies:
          flutter:
            sdk: flutter
          flutter_launcher_icons: ^0.9.2
        
        
        
          # The following adds the Cupertino Icons font to your application.
          # Use with the CupertinoIcons class for iOS style icons.
          cupertino_icons: ^1.0.2
        dev_dependencies:
          flutter_test:
            sdk: flutter
        flutter_icons:
          image_path: "assets/Vector-2.png"
          android: "launcher_icon"
          ios: true
          remove_alpha_ios: true
        
        
        # For information on the generic Dart part of this file, see the
        # following page: https://dart.dev/tools/pub/pubspec
        
        # The following section is specific to Flutter.
        flutter:
        
          # The following line ensures that the Material Icons font is
          # included with your application, so that you can use the icons in
          # the material Icons class.
          uses-material-design: true
          assets:
            - assets/
            - assets/Vector-2.png
        
          # To add assets to your application, add an assets section, like this:
          # assets:
          #   - images/a_dot_burr.jpeg
          #   - images/a_dot_ham.jpeg
        
          # An image asset can refer to one or more resolution-specific "variants", see
          # https://flutter.dev/assets-and-images/#resolution-aware.
        
          # For details regarding adding assets from package dependencies, see
          # https://flutter.dev/assets-and-images/#from-packages
        
          # To add custom fonts to your application, add a fonts section here,
          # in this "flutter" section. Each entry in this list should have a
          # "family" key with the font family name, and a "fonts" key with a
          # list giving the asset and other descriptors for the font. For
          # example:
          # fonts:
          #   - family: Schyler
          #     fonts:
          #       - asset: fonts/Schyler-Regular.ttf
          #       - asset: fonts/Schyler-Italic.ttf
          #         style: italic
          #   - family: Trajan Pro
          #     fonts:
          #       - asset: fonts/TrajanPro.ttf
          #       - asset: fonts/TrajanPro_Bold.ttf
          #         weight: 700
          #
          # For details regarding fonts from package dependencies,
          # see https://flutter.dev/custom-fonts/#from-packages

我可能有一个使用flutter_gen包的解决方案。我设法将其简化为在依赖包的pubspec.yaml文件中没有这种结构，在您的情况下是包A：

flutter_gen:
  assets:
    outputs:
      package_parameter_enabled: true

添加此内容后，我可以看到我的PDF文件。[这是我的用例]