Add es6-promise for Internet Explorer

Plus it is required by the fetch polyfill, see
https://github.com/github/fetch#installation

Also see https://github.com/github/fetch/issues/114

This fixes the undefined Promise error in internet explorer 11, due to
Promises not being supported in Internet explorer and other browsers.

See http://caniuse.com/#feat=promises

Bug: Issue 4308
Change-Id: I93df9c89a9a98059292038dffe1655c3f4d503c5

lib/js/BUCK

@@ -99,6 +99,14 @@ bower_component(
   sha1 = '849ad3ee7c77506548b7b5db603a4e150b9431aa',
 )
 
+bower_component(
+  name = 'es6-promise',
+  package = 'stefanpenner/es6-promise',
+  version = '3.2.2',
+  license = 'es6-promise',
+  sha1 = 'de2eda2fdd744994180a0df03d2b5a936fcb1e70',
+)
+
 bower_component(
   name = 'fetch',
   package = 'fetch',