Made flutter tools serve metadata produced by the frontend server:

pass --experimental-emit-metadata flag to flutter engine
store and serve metadata from web asset server
store and serve merged metadata from web asset server
add tests to verify that metadata is served from memory
update dwds version so it can read metadata
configure dwds to read metadata from file
Prerequisite changes (landed):

sdk: https://dart-review.googlesource.com/c/sdk/+/150181
flutter engine: flutter/engine#19168
webdev:dart-lang/webdev#1064