Increasing Serial Buffer Length to 256

I am trying to decode the IR protocol of an AC remote and realize that the signal is getting truncated because the same hex code appears for different button presses. I need to increase the Serial buffer length to 256.
I am using Arduino 1.8.3.
Here’s what I did: I went to
C:\Users\Downloads\arduino-1.8.3-windows\arduino-1.8.3\hardware\arduino\avr\cores\arduino
and made a replica of the destination folder with the name arduino_256_serialbuffer. Then I went to

C:\Users\Downloads\arduino-1.8.3-windows\arduino-1.8.3\hardware\arduino\avr\cores\arduino_256_serialbuffer\USBAPI.h

and changed the following piece of code :

#ifndef SERIAL_BUFFER_SIZE
#if ((RAMEND - RAMSTART) < 1023)
#define SERIAL_BUFFER_SIZE 16
#else
#define SERIAL_BUFFER_SIZE 64
#endif
#endif
#if (SERIAL_BUFFER_SIZE>256)
#error Please lower the CDC Buffer size
#endif

to this:

#ifndef SERIAL_BUFFER_SIZE
#if ((RAMEND - RAMSTART) < 1023)
#define SERIAL_BUFFER_SIZE 16
#else
#define SERIAL_BUFFER_SIZE 256
#endif
#endif
#if (SERIAL_BUFFER_SIZE>256)
#error Please lower the CDC Buffer size
#endif

Then I went back to :
C:\Users\Downloads\arduino-1.8.3-windows\arduino-1.8.3\hardware\arduino\avr\boards

and added the following the code block:

############################

uno256.name=Arduino Uno (256 Serial Buffer)

uno256.upload.protocol=arduino
uno256.upload.maximum_size=32256
uno256.upload.speed=115200
uno256.bootloader.low_fuses=0xff
uno256.bootloader.high_fuses=0xde
uno256.bootloader.extended_fuses=0x05
uno256.bootloader.path=optiboot
uno256.bootloader.file=optiboot_atmega328.hex
uno256.bootloader.unlock_bits=0x3F
uno256.bootloader.lock_bits=0x0F
uno256.build.mcu=atmega328p
uno256.build.f_cpu=16000000L
uno256.build.core=arduino_256_serialbuf
uno256.build.variant=standard

########################

After this, in the Tools menu of my Arduino IDE, I got an option for Arduino 256 Serial Buffer.
But it does not seem to compile correctly.

Error message:

fatal error: Arduino.h : No such file or directory

compilation terminated.

Why does it not work?

Does nobody have an answer?