• rev the dart sdk and package:http dep

This brings in the latest Dart sdk dev commit, and updates the repo to account for a change in the package:http dep (it changed from a single package repo to a mono-repo).

I find it useful in case these rolls have to be looked at closely for potentially causing some regressions to include the list of dart sdk changes that ended up in the roll into the PR description. Basically the output of $ git log --oneline a4d8cce0f7e72c440243f11a15184e0e677b6cda..5beb49ce6f29dfb3e83c375f31bed0a1cedabd04 | cat

5beb49ce6f2 Version 2.18.0-189.0.dev
d6e5a0fc49c [analysis_server] Support Flutter's "wrap" assists in variable declarations
f57e8cce799 [cfe] Move closureContext to InferenceVisitor
9e32094411f [CFE] Fix unevaluated constants not correctly substituting types leading to crash on serialization
0efe9a56355 [CFE] Fixup things found by weekly bot runs #94 and #95