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This CL migrates most of import maps resolution tests by
observing the resolution results by intercepting
module script requests by a service worker.

This CL also introduces `useInternalMethods` flag to clarify
the tests still requiring internal methods.

Bug: 1026809
Change-Id: I16c2a87bb67b530dc97b1631f6968b2d3bafdac6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2457526
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Domenic Denicola <domenic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818366}