File size reporter on Windows

Created by: fiddep

Is this a bug report?

Yes

Did you try recovering your dependencies?

Yes

Which terms did you search for in User Guide?

windows, build, size

Environment

Environment:
  OS:  Windows 10
  Node:  10.9.0
  Yarn:  1.9.4
  npm:  6.2.0
  Watchman:  Not Found
  Xcode:  N/A
  Android Studio:  Not Found

Packages: (wanted => installed)
  react: ^16.4.2 => 16.4.2
  react-dom: ^16.4.2 => 16.4.2
  react-scripts: 1.1.5 => 1.1.5

Steps to Reproduce

create a fresh app
run yarn build
add stuff to any file
run yarn build

Expected Behavior

the output would show that the file size has increased if a previous build was present

Actual Behavior

Nothing else except the current filesize is show Before: http://prntscr.com/kndf9p After: http://prntscr.com/kndit2

Possible Solution

The issue seems to be related to Windows and its path delimiter. Adding .replace(/\\/g, '/') in FileSizeReporter Solves the problem and don't seem to cause any issues on other OS

Would happily submit a PR for this