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

[DartBot]

Originally opened as dart-lang/sdk#18746

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.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/8631949?v=3" align="left" width="48" height="48"hspace="10"> Comment by floitschG

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

[DartBot]

<img src="https://avatars.githubusercontent.com/u/17034?v=3" align="left" width="48" height="48"hspace="10"> Comment by kevmoo

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

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="48" height="48"hspace="10"> Comment by nex3

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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.