anchor.diff:      _enterScope(closureScope);
     }
   
  -  List<ir.Primitive> buildFunctionHeader(Iterable<Local> parameters,
  -                                        {ClosureScope closureScope,
  -                                         ClosureEnvironment env}) {
  +  void buildFunctionHeader(Iterable<Local> parameters,
  +                           {ClosureScope closureScope,
  +                            ClosureEnvironment env}) {
       _createThisParameter();
       _enterClosureEnvironment(env);
       _enterScope(closureScope);
       parameters.forEach(_createFunctionParameter);
  -    return _parameters;
     }
   
     /// Creates a parameter for [local] and adds it to the current environment.
     ir.Parameter createLocalParameter(Local local) {
       ir.Parameter parameter = new ir.Parameter(local);
  -    _parameters.add(parameter);
       environment.extend(local, parameter);
       return parameter;
     }
  
positive.code: 
    void _createFunctionParameter(Local parameterElement) {
      ir.Parameter parameter = new ir.Parameter(parameterElement);
      _parameters.add(parameter);
      state.functionParameters.add(parameter);
      ClosureLocation location = jsState.boxedVariables[parameterElement];
      if (location != null) {
  
negative.code: 
    void _createFunctionParameter(Local parameterElement) {
      ir.Parameter parameter = new ir.Parameter(parameterElement);
      if (isInMutableVariable(parameterElement)) {
        state.functionParameters.add(getMutableVariable(parameterElement));
      } else {
  