Fix editor warnings in http_server and mime.

pkg/http_server/lib/src/http_multipart_form_data_impl.dart

 // Copyright (c) 2012, 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 http_server;
 
 
 class _HttpMultipartFormData extends Stream implements HttpMultipartFormData {
   final ContentType contentType;
   final HeaderValue contentDisposition;
@@ skipping 43 matching lines @@
   bool get isText => _isText;
   bool get isBinary => !_isText;
 
   static HttpMultipartFormData parse(MimeMultipart multipart) {
     var type;
     var encoding;
     var disposition;
     var remaining = new Map<String, String>();
     for (String key in multipart.headers.keys) {
       switch (key) {
-        case HttpHeaders.CONTENT_TYPE:
+        case 'content-type':
           type = ContentType.parse(multipart.headers[key]);
           break;
 
         case 'content-transfer-encoding':
           encoding = HeaderValue.parse(multipart.headers[key]);
           break;
 
         case 'content-disposition':
           disposition = HeaderValue.parse(multipart.headers[key]);
           break;
@@ skipping 54 matching lines @@
     while ((amp = input.indexOf('&', offset)) >= 0) {
       buffer.write(input.substring(offset, amp));
       int end = input.indexOf(';', amp);
       parse(amp, end);
       offset = end + 1;
     }
     buffer.write(input.substring(offset));
     return buffer.toString();
   }
 }