Fix type warnings in API and helpers.

sdk/lib/io/http_headers.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;
 
 class _HttpHeaders implements HttpHeaders {
   _HttpHeaders(String this.protocolVersion)
       : _headers = new Map<String, List<String>>();
 
@@ skipping 189 matching lines @@
     } else {
       return null;
     }
   }
 
   void set contentType(ContentType contentType) {
     _checkMutable();
     _set(HttpHeaders.CONTENT_TYPE, contentType.toString());
   }
 
-  void _add(String name, Object value) {
+  void _add(String name, value) {
     var lowerCaseName = name.toLowerCase();
     // TODO(sgjesse): Add immutable state throw HttpException is immutable.
     if (lowerCaseName == HttpHeaders.CONTENT_LENGTH) {
       if (value is int) {
         contentLength = value;
       } else if (value is String) {
         contentLength = int.parse(value);
       } else {
         throw new HttpException("Unexpected type for header named $name");
       }
@@ skipping 22 matching lines @@
     } else if (lowerCaseName == HttpHeaders.IF_MODIFIED_SINCE) {
       if (value is DateTime) {
         ifModifiedSince = value;
       } else if (value is String) {
         _set(HttpHeaders.IF_MODIFIED_SINCE, value);
       } else {
         throw new HttpException("Unexpected type for header named $name");
       }
     } else if (lowerCaseName == HttpHeaders.HOST) {
       if (value is String) {
-        int pos = (value as String).indexOf(":");
+        int pos = value.indexOf(":");
         if (pos == -1) {
           _host = value;
           _port = HttpClient.DEFAULT_HTTP_PORT;
         } else {
           if (pos > 0) {
-            _host = (value as String).substring(0, pos);
+            _host = value.substring(0, pos);
           } else {
             _host = null;
           }
           if (pos + 1 == value.length) {
             _port = HttpClient.DEFAULT_HTTP_PORT;
           } else {
             try {
               _port = int.parse(value.substring(pos + 1));
             } on FormatException catch (e) {
               _port = null;
@@ skipping 501 matching lines @@
 
   String name;
   String value;
   DateTime expires;
   int maxAge;
   String domain;
   String path;
   bool httpOnly = false;
   bool secure = false;
 }