Add handling of HTTP '100 Continue' intermediate response

tests/standalone/io/http_100_continue.dart

+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import "dart:async";
+import 'dart:convert';
+import "dart:io";
+
+import "package:expect/expect.dart";
+
+void test(responseBytes, bodyLength) async {
+  fullRequest(bytes) {
+    var len = bytes.length;
+    return len > 4 &&
+           bytes[len - 4] == 13 &&
+           bytes[len - 3] == 10 &&
+           bytes[len - 2] == 13 &&
+           bytes[len - 1] == 10;
+  }
+
+  handleSocket(socket) async {
+    var bytes = [];
+    await for (var data in socket) {
+        bytes.addAll(data);
+        if (fullRequest(bytes)) {
+          socket.add(responseBytes);
+          socket.close();
+        }
+    }
+  }
+
+  var server = await ServerSocket.bind('127.0.0.1', 0);
+  server.listen(handleSocket);
+
+  var client = new HttpClient();
+  var request =
+      await client.getUrl(Uri.parse('http://127.0.0.1:${server.port}/'));
+  var response = await request.close();
+  Expect.equals(response.statusCode, 200);
+  Expect.equals(bodyLength,
+                (await response.fold([], (p, e) => p..addAll(e))).length);
+  server.close();
+}
+
+main() {
+  var r1 = '''
+HTTP/1.1 100 Continue\r
+\r
+HTTP/1.1 200 OK\r
+\r
+''';
+
+  var r2 = '''
+HTTP/1.1 100 Continue\r
+My-Header-1: hello\r
+My-Header-2: world\r
+\r
+HTTP/1.1 200 OK\r
+\r
+''';
+
+  var r3 = '''
+HTTP/1.1 100 Continue\r
+\r
+HTTP/1.1 200 OK\r
+Content-Length: 2\r
+\r
+AB''';
+
+  test(ASCII.encode(r1), 0);
+  test(ASCII.encode(r2), 0);
+  test(ASCII.encode(r3), 2);
+}