build: Use String#slice instead of discouraged String#substr

Aside from the confusion and differences between substr() and
substring() and IE8 bugs with substr(), substr() was removed from
the spec as of ECMAScript 5. It's been standardised in the
optional Annex B section of ES5.

Change-Id: I221ef6ae6956ce20dd9bb74510500f747d04c3b1
Merge "build: Use String#slice instead of discouraged String#substr"