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

dart/pkg/http_base/test/http_base_html_test.dart

+library http_base.http_base_html_test;
+
+import 'dart:html';
+import 'dart:async';
+import 'dart:convert';
+
+import 'package:http_base/http_base_html.dart';
+import 'package:unittest/unittest.dart';
+
+main() {
+  test('http-client', () {
+    var uri = Uri.parse(window.location.href).resolve('/echo');
+
+    var client = new Client();
+    var body = (new StreamController()
+        ..add(UTF8.encode('my-data'))
+        ..close()).stream;
+    var request = new RequestImpl('POST', uri, body: body);
+    client(request).then(expectAsync((response) {
+      expect(response.statusCode, equals(200));
+      response.read()
+          .transform(UTF8.decoder).join('').then(expectAsync((data) {
+        expect(data, equals('my-data'));
+      }));
+    }));
+  });
+}