Restructure http/browser_client.dart in http package to avoid dart:mirrors usage (?)

[DartBot]

Blocked by dart-lang/sdk#24581

[DartBot]

<img src="https://avatars.githubusercontent.com/u/1047298?v=3" align="left" width="48" height="48"hspace="10"> Comment by Scorpiion

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To be clear, a simple Hello World app importing package:http/browser_client.dart gives this:

* Your app imports dart:mirrors via:

• main.dart => package:http => dart:mirrors

[DartBot]

<img src="https://avatars.githubusercontent.com/u/4865287?v=3" align="left" width="48" height="48"hspace="10"> Comment by lrhn

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Removed Type-Defect label.
Added Type-Enhancement, Pkg-Http, Area-Pkg, Triaged 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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The warning sucks, but in practice the usage of mirrors in the http package is harmless. It's only used to determine that dart:io is unavailable, and adds essentially no size overhead to the compiled output.

I'd love to remove mirrors from the package, but right now the language just doesn't support it while maintaining the same API.

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Marked this as being blocked by dart-lang/sdk#6943.

[polux]

The usage of mirrors makes it impossible to use this lib with flutter (which disables mirrors).

[sethladd]

Yes. We had to fork http here: https://github.com/flutter/flutter/blob/master/packages/flutter/lib/http.dart

[nex3]

Disabling mirrors makes sense, but making programs statically invalid just by importing dart:mirrors seems like overkill. Why not just have createMirrorSystem() throw an error? Then you could use HTTP and any other libraries that use similar workarounds.

[nex3]

I'm merging this as a duplicate of #1.