Fix per breaking change to HttpRequest and HttpClientResponse (dart-l…

…ang#2335)

A recent change to the Dart SDK updated `HttpRequest` and
`HttpClientResponse` from implementing `Stream<List<int>>` to
implementing `Stream<Uint8List>`.

This forwards-compatible change prepares for that SDK breaking
change by casting the Stream to `List<int>` before transforming
it.

dart-lang/sdk#36900

release build_runner 1.6.1 (dart-lang#2330)

Fix hot restart for apps under lib (dart-lang#2309)

use a polling watcher on windows again (dart-lang#2307)

handle symlink creation failures, link to dev mode docs (dart-lang#2303)

set the exit code in the generated build script (dart-lang#2301)

Also check the root module for platform compatibility (dart-lang#2300)

allow a custom reader to be passed to testBuilder (dart-lang#2297)

update build_web_compilers 1.x to allow build_config 0.4.x (dart-lang…

…#2293)

update build_modules 1.x to support build_config 0.4.x (dart-lang#2294)