Add fetch polyfill for use in all supported browsers

Additionally fix a bug in bower2buck.py where it would
bail when a license is not specified in a Bower package
info response.

[1] http://caniuse.com/#feat=fetch
[2] http://github.github.io/fetch/

Change-Id: Ibf754aab98bab6e34b7a9fb5d6c9801bb612f5d5

polygerrit-ui/BUCK

@@ -3,6 +3,7 @@ include_defs('//lib/js.defs')
 bower_components(
   name = 'polygerrit_components',
   deps = [
+    '//lib/js:fetch',
     '//lib/js:iron-ajax',
     '//lib/js:iron-autogrow-textarea',
     '//lib/js:iron-dropdown',
@@ -11,5 +12,6 @@ bower_components(
     '//lib/js:iron-selector',
     '//lib/js:page',
     '//lib/js:polymer',
+    '//lib/js:promise-polyfill',
   ],
 )