use package:web for the grpc-web example

[devoncarew]

• use package:web for the grpc-web example
• some work towards update use of deprecated dart:html apis #756

[github-actions]

PR Health

Breaking changes ⚠️

Package	Change	Current Version	New Version	Needed Version	Looking good?
grpc	Breaking	4.0.1	4.0.2-wip	5.0.0 Got "4.0.2-wip" expected >= "5.0.0" (breaking changes)	⚠️

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Changelog Entry ❗

Package	Changed Files
package:grpc	example/grpc-web/pubspec.yaml lib/src/shared/status.dart

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Coverage ✔️

File	Coverage
lib/src/shared/status.dart	💚 59 %

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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

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[devoncarew]

Hmm, it looks like there are already some in-flight PRs to address #756 (https://github.com/grpc/grpc-dart/pulls); we should probably prefer to land some combination of them.