Support a user-supplied dart:io client in pkg/http.

pkg/http/CHANGELOG.md

+## 0.11.1
+
+* Allow a user-supplied `dart:io` `HttpClient` to be passed in to the
+  `IOClient` constructor.
+
 ## 0.11.0+1
 
 * Fix a bug in handling errors in decoding XMLHttpRequest responses for
   `BrowserClient`.
 
 ## 0.11.0
 
 * The package no longer depends on `dart:io`. The `BrowserClient` class in
   `package:http/browser_client.dart` can now be used to make requests on the
   browser.
 
 * Change `MultipartFile.contentType` from `dart:io`'s `ContentType` type to
   `http_parser`'s `MediaType` type.
 
 * Exceptions are now of type `ClientException` rather than `dart:io`'s
   `HttpException`.
 
 ## 0.10.0
 
 * Make `BaseRequest.contentLength` and `BaseResponse.contentLength` use `null`
   to indicate an unknown content length rather than -1.
 
 * The `contentLength` parameter to `new BaseResponse` is now named rather than
   positional.
 
 * Make request headers case-insensitive.
 
 * Make `MultipartRequest` more closely adhere to browsers' encoding conventions.