anchor.diff:  
     R build() {
       assert(implementationBuilders.isEmpty);
  +    canAddImplementationBuilders = true;
       forEach((String name, Builder builder) {
         do {
           buildBuilder(builder);
  
positive.code:   /// for example, [addClass] is called.
    DeclarationBuilder<T> currentDeclaration;
  
    SourceLibraryBuilder(this.loader, Uri fileUri)
        : fileUri = fileUri,
          super(fileUri) {
  
negative.code:       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]);
    }
  
  