Bug 1614503 [wpt PR 21707] - [Import Maps] Make errors block the whole resolution...

Bug 1614503 [wpt PR 21707] - [Import Maps] Make errors block the whole resolution, Blink-side, a=testonly

Automatic update from web-platform-tests
[Import Maps] Make errors block the whole resolution, Blink-side

Reflects https://github.com/WICG/import-maps/pull/205.

This CL updates tests from #205.

To match the behavior with the updated spec,
this CL turns non-String values into `null` entries
(i.e. make whole resolution fail without further fallback),
instead of ignoring such entries.
Other aspects were already conformant with the updated spec
(i.e. weren't matching with the spec before #205).

This CL updates (test-only) import maps serialization code
so that it matches with the reference implementation, i.e.
dump `null` entries as `null` instead of `[]`.

This CL also updates spec comments.

Bug: 990561, https://github.com/WICG/import-maps/issues/184
Change-Id: Ifa2d04bf20fcef5575c14d135c328730ea09c454
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2037012
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740847}

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wpt-commits: 8767abfffb790e4928ac9433282928caa2357ee8
wpt-pr: 21707