Reland "[web] Accepts assetBase through JS config. (#40615)"

[ditman]

This PR relands #40615, by:

• Reverts commit 6cc2f9b, PR Revert "[web] Accepts assetBase through JS config. (#40615)" #40670.
• Solves an issue with the library_annotations lint that was added to the repo:

   info - test/engine/assets_test.dart:5:1 - This annotation must
     be attached to a library directive. Try attaching library
     annotations to library directives. - library_annotations

Fixes: flutter/flutter#122987

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

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