anchor.diff:        addBuilder(name, builder, charOffset);
         buildBuilder(builder);
       }
  +    canAddImplementationBuilders = false;
       return null;
     }
   
  +  /// Used to add implementation builder during the call to [build] above.
  +  /// Currently, only anonymous mixins are using implementation builders (see
  +  /// [KernelMixinApplicationBuilder]
  +  /// (../kernel/kernel_mixin_application_builder.dart)).
     void addImplementationBuilder(String name, Builder builder, int charOffset) {
  +    assert(canAddImplementationBuilders, "$uri");
       implementationBuilders.add([name, builder, charOffset]);
     }
   
  
positive.code:   /// for example, [addClass] is called.
    DeclarationBuilder<T> currentDeclaration;
  
    SourceLibraryBuilder(this.loader, Uri fileUri)
        : fileUri = fileUri,
          super(fileUri) {
  
negative.code: 
    R build() {
      assert(implementationBuilders.isEmpty);
      canAddImplementationBuilders = true;
      forEach((String name, Builder builder) {
        do {
          buildBuilder(builder);
  