http library should redefine some of the enums from 'dart:io'

[DartBot]

This issue was originally filed by ovan...@gmail.com

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Now that the http library supports both browser and console contexts, libraries intending to use the library to provide requests are unable to use some of the useful enums defined in 'dart:io', (specific to my use case, HttpHeaders and HttpStatus).

It would be useful if the http library could redefine or reexport some of these classes.

[floitschG]

Removed Type-Defect label.
Added Type-Enhancement, Area-Library, Triaged labels.

[kevmoo]

Removed Area-Library label.
Added Area-Pkg, Pkg-Http labels.

[nex3]

I don't think these enums add a lot of value compared to the verbosity of using them. The HttpHeaders enum names already encode the names of the headers, and the HTTP response codes are often known better numerically than by their names.

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Added NotPlanned label.

[DartBot]

This issue has been moved to dart-lang/http#18.