Ionic2: How to get a absolute path to www/build directory

I have tried using the FileOpener class of ionic-native and am able to open files only when I specify full path to the file but that full path keeps on changing with device so I need an absolute path to www/build directory.I have already read this thread but it is not of good help to me. I want to open a pdf file stored locally in build folder with the native pdf reader application present in the device.

.ts file
openPDF(url) {,'application/pdf'); }

.html file
<button (click)="openPDF('build/pdf/AssignmentB.pdf')">Open pdf</button>

package.json file

Note:- This is my first time I am interacting in forums so sorry if I have missed out something.