[ES6 modules] Fix obsolete assertions and add missing bailout to FetchDescendants

This CL :
1. Removes oesolete assertions after
crrev.com/e123c9108586cd8386edf8df90af739ce84ae3a4

2. Adds missing !HasValidContext bailout in FetchDescendants

Bug: 732765, 594639
Change-Id: I806be731f528a1e8c1a2a9ac7f882dfaaf98e2ba
Reviewed-on: https://chromium-review.googlesource.com/535257
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#479341}
2 files changed