Don't use direct conditional exports for sdk libraries

[juancastillo0]

Fixes https://github.com/dint-dev/universal_html/issues/48.

Hi, i was having the same issue and this changes fixed it for me, more info in https://github.com/dart-lang/build/issues/3173. You already had the src/_sdk directory with all dart:<library>, so I exported those instead of directly exporting the dart sdk libraries.

Please let me know if there is anything more I can do. Thanks for your work!

[rokk4]

Please merge this.

[terrier989]

Thanks for the PR! 👍