Add implementations for Headers, Request, Response and dart:io/dart:html clients to http_base

dart/pkg/http_base/test/http_base_io_test.dart

+library http_base.http_base_io_test;
+
+import 'dart:async';
+import 'dart:io';
+import 'dart:convert';
+
+import 'package:http_base/http_base_io.dart';
+import 'package:unittest/unittest.dart';
+
+main() {
+  test('io-client', () {
+    HttpServer.bind('127.0.0.1', 0).then(expectAsync((HttpServer server) {
+      server.first.then(expectAsync((request) {
+        expect(request.method, equals('POST'));
+        expect(request.headers.value('foo'), equals('bar'));
+
+        return request.fold([], (buf, data) => buf..addAll(data)).then((data) {
+          request
+              ..response.statusCode = 201
+              ..response.headers.set('foo', ['foo', 'bar'])
+              ..response.add(data)
+              ..response.close();
+        });
+      })).whenComplete(() => server.close());
+
+      var client = new Client();
+      var uri = Uri.parse('http://127.0.0.1:${server.port}/');
+      var headers = new HeadersImpl({'foo' : 'bar'});
+      var body = (new StreamController()
+          ..add(UTF8.encode('my-data'))
+          ..close()).stream;
+
+      var request = new RequestImpl('POST', uri, headers: headers, body: body);
+      client(request).then(expectAsync((response) {
+        expect(response.statusCode, equals(201));
+        // NOTE: dart:io joins multiple values with ", ".
+        expect(response.headers['foo'], equals('foo, bar'));
+        expect(response.headers.getMultiple('foo').toList(),
+               equals(['foo','bar']));
+
+        response.read()
+            .transform(UTF8.decoder).join('').then(expectAsync((data) {
+          expect(data, equals('my-data'));
+        }));
+      }));
+    }));
+  });
+}