anchor.diff:  
     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) {
  
positive.code:     _enterScope(closureScope);
    }
  
    List<ir.Primitive> 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;
    }
  
negative.code:   // TODO(johnniwinther): Make these field final and remove the default values
    // when [IrBuilder] is a property of [IrBuilderVisitor] instead of a mixin.
  
    IrBuilderDelimitedState state;
  
    /// A map from variable indexes to their values.
  