Make package:http/http.dart support all platforms

[jakemac53]

Fixes #22

Adds config specific imports to make package:http/http.dart support all platforms, and allow the Client factory constructor to return a valid Client for the web platform.

This should eliminate almost all need for the platform specific imports for consumers, although it does also add the io_client.dart public import.

Passes presubmit internally, with edits in only 3 files (they use the IoClient constructor directly, and pass in an HttpClient). Externally build_runner now supports config specific imports as well, I am currently working on validating everything works as expected there with this change.

New Features

• The regular Client factory constructor is now usable anywhere that dart:io
  or dart:html are available, and will give you an IoClient or
  BrowserClient respectively.
• The package:http/http.dart import is now safe to use on the web (or
  anywhere that either dart:io or dart:html are available).

Breaking Changes

• In order to use or reference the IoClient directly, you will need to import
  the new package:http/io_client.dart import. This is typically only necessary
  if you are passing a custom HttpClient instance to the constructor, in which
  case you are already giving up support for web.

[zoechi]

The environment constraint lower limit is correct to support conditional imports?

[jakemac53]

The environment constraint lower limit is correct to support conditional imports?

Technically, yes. It has been supported by the language and vm/dart2js for some time. We only recently actually had support in build systems and things, but we can't express that dependency unfortunately (ideally, we would be able to say if you have a dependency on build_runner, it must be >x.x.x (which supports these imports), but we can't express that).

[nex3]

I'm swamped with Sass work and it looks like Nate has this review in hand, so I'm going to remove myself.

[natebosch]

I think this might also close out #22