* Revert "Include _http into sky_engine libraries for analyzer (#4154)"

This reverts commit b930f107.

It is dependent upon the Dart roll, which breaks the engine.

* Revert "Roll dart to ade37f931e90b0fdb8fe16d6bf6f089545da55b6 (#4151)"

This reverts commit 6f1264fb.

This Dart roll breaks the engine build on Travis.