Fix warnings and hints in the SDK

sdk/lib/io/http_impl.dart

 // Copyright (c) 2013, 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.
 
 part of dart.io;
 
 const int _OUTGOING_BUFFER_SIZE = 8 * 1024;
 
 class _HttpIncoming extends Stream<List<int>> {
   final int _transferLength;
@@ skipping 922 matching lines @@
   Future writeHeaders({bool drainRequest: true, bool setOutgoing: true}) {
     Future write() {
       try {
         outbound._writeHeader();
       } catch (_) {
         // Headers too large.
         return new Future.error(new HttpException(
             "Headers size exceeded the of '$_OUTGOING_BUFFER_SIZE'"
             " bytes"));
       }
+      return null;
     }
     if (headersWritten) return null;
     headersWritten = true;
     Future drainFuture;
     bool isServerSide = outbound is _HttpResponse;
     bool gzip = false;
     if (isServerSide) {
       var response = outbound;
       if (response._httpRequest._httpServer.autoCompress &&
           outbound.bufferOutput &&
@@ skipping 949 matching lines @@
 
   _ProxyCredentials _findProxyCredentials(_Proxy proxy,
                                           [_AuthenticationScheme scheme]) {
     // Look for credentials.
     var it = _proxyCredentials.iterator;
     while (it.moveNext()) {
       if (it.current.applies(proxy, scheme)) {
         return it.current;
       }
     }
+
+    return null;
   }
 
   void _removeCredentials(_Credentials cr) {
     int index = _credentials.indexOf(cr);
     if (index != -1) {
       _credentials.removeAt(index);
     }
   }
 
   void _removeProxyCredentials(_Credentials cr) {
@@ skipping 894 matching lines @@
   const _RedirectInfo(this.statusCode, this.method, this.location);
 }
 
 String _getHttpVersion() {
   var version = Platform.version;
   // Only include major and minor version numbers.
   int index = version.indexOf('.', version.indexOf('.') + 1);
   version = version.substring(0, index);
   return 'Dart/$version (dart:io)';
 }