anchor.diff:  
     void _createFunctionParameter(Local parameterElement) {
       ir.Parameter parameter = new ir.Parameter(parameterElement);
  +    _parameters.add(parameter);
       if (isInMutableVariable(parameterElement)) {
         state.functionParameters.add(getMutableVariable(parameterElement));
       } else {
  
positive.code: 
    void _createFunctionParameter(Local parameterElement) {
      ir.Parameter parameter = new ir.Parameter(parameterElement);
      state.functionParameters.add(parameter);
      ClosureLocation location = jsState.boxedVariables[parameterElement];
      if (location != null) {
  
negative.code:   // TODO(johnniwinther): Make these field final and remove the default values
    // when [IrBuilder] is a property of [IrBuilderVisitor] instead of a mixin.
  
    final List<ir.Parameter> _parameters = <ir.Parameter>[];
  
    IrBuilderDelimitedState state;
  
    /// A map from variable indexes to their values.
  