Use internal libc bindings

[brianquinlan]

Use an internal libc shim instead of package:stdlibc. package:stdlibc does not support Android, intel macOS and arm linux.

For an explanation of the approach, see:
https://github.com/dart-lang/labs/pull/231/files#diff-d7066106d7deb29324ec6e4232d1dc307e2aa74333e15bc520a5b253c80e4fecR5

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[dcharkes]

nit: newline

[brianquinlan]

The json pretty printer doesn't like newlines for some reason.

[dcharkes]

Nit: As best practice, I've adopted having a tool/generate.dart that does all the generators in the project with a --set-exit-if-changed param. That way you can run all of them locally easily as well pre-commit.

[brianquinlan]

I'll fix this in another PR