update: Migrate off legacy JS/HTML apis

[minoic]

The purpose of this PR is to fix the issue mentioned in #715, I have test both unary/server side stream requests on canvaskit/wasm platform.Thanks to @hyunw55 `s comment in #715 (comment).

Main updates include:

1. add web v1.0.0 package and XMLHttpRequest to replace html package.
2. use dart.library.io to determine the current platform.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: minoic / name: minoic (2cda92b, 05756b7, b96174d, c424253, 4621ac7, 3d9ee25)

[aran]

@kevmoo do you happen to know who would be the right maintainers to look at this?

[kevmoo]

Thanks for chiming in here @aran
And thanks for the PR @minoic

I'll work to find a reviewer!

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
grpc	Non-Breaking	4.0.2-wip	4.0.2-wip	4.0.2-wip	✔️

Changelog Entry ✔️

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

Coverage ⚠️

File	Coverage
lib/grpc_or_grpcweb.dart	💚 57 %
lib/src/client/transport/xhr_transport.dart	💔 Not covered

This check for test coverage is informational (issues shown here will not fail the PR).

This check can be disabled by tagging the PR with skip-coverage-check.

API leaks ⚠️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbols
grpc	Any $1.Duration ServerHandler

This check can be disabled by tagging the PR with skip-leaking-check.

Package publish validation ✔️

Package	Version	Status
package:grpc	4.0.2-wip	WIP (no publish necessary)

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[kevmoo]

We published a new pkg:web with those HTTP helpers

We need to rebase this fix, too

[kevmoo]

looks like we need to do another merge here

[aran]

@kevmoo see #747 for an attempt to push this forward.