code: public FunctionType createFunctionTypeWithVarArgs(
        JSType returnType, JSType... parameterTypes) {
      return createFunctionType(
          returnType, createParametersWithVarArgs(parameterTypes));
    }
docs: Creates a function type. The last parameter type of the function is
  considered a variable length argument.
  
  @param returnType the function's return type
  @param parameterTypes the parameters' types