anchor.diff:          new DartIrBuilder(DART_CONSTANT_SYSTEM,
                             element,
                             // TODO(johnniwinther): Supported closure variables.
  -                          new NullCapturedVariableInfo()),
  +                          new NullCapturedVariables()),
           () {
         irBuilder.buildFunctionHeader(
             constructor.parameters.map(converter.convertElement));
  
positive.code:   }
  }
  
  class NullCapturedVariableInfo extends DartCapturedVariableInfo {
    Iterable get capturedVariables => const [];
  }
  
negative.code:         new DartIrBuilder(DART_CONSTANT_SYSTEM,
                            element,
                            // TODO(johnniwinther): Supported closure variables.
                            new NullCapturedVariables()),
          () {
        irBuilder.buildFunctionHeader(
            function.parameters.map(converter.convertElement));
  