Preserve '\' in filenames for uploads, as Windows/IE contains them.

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 464 matching lines @@
   String _value;
   _UnmodifiableMap<String, String> _parameters;
 
   _HeaderValue([String this._value = "", Map<String, String> parameters]) {
     if (parameters != null) {
       _parameters =
           new _UnmodifiableMap(new Map<String, String>.from(parameters));
     }
   }
 
-  static _HeaderValue parse(String value, {parameterSeparator: ";"}) {
+  static _HeaderValue parse(String value,
+                            {parameterSeparator: ";",
+                             preserveBackslash: false}) {
     // Parse the string.
     var result = new _HeaderValue();
-    result._parse(value, parameterSeparator);
+    result._parse(value, parameterSeparator, preserveBackslash);
     return result;
   }
 
   String get value => _value;
 
   void _ensureParameters() {
     if (_parameters == null) {
       _parameters = new _UnmodifiableMap(new Map<String, String>());
     }
   }
@@ skipping 10 matching lines @@
       _parameters.forEach((String name, String value) {
         sb.write("; ");
         sb.write(name);
         sb.write("=");
         sb.write(value);
       });
     }
     return sb.toString();
   }
 
-  void _parse(String s, String parameterSeparator) {
+  void _parse(String s, String parameterSeparator, bool preserveBackslash) {
     int index = 0;
 
     bool done() => index == s.length;
 
     void skipWS() {
       while (!done()) {
         if (s[index] != " " && s[index] != "\t") return;
         index++;
       }
     }
@@ skipping 32 matching lines @@
         }
         return s.substring(start, index).toLowerCase();
       }
 
       String parseParameterValue() {
         if (s[index] == "\"") {
           // Parse quoted value.
           StringBuffer sb = new StringBuffer();
           index++;
           while (!done()) {
+            print(s[index]);
             if (s[index] == "\\") {
               if (index + 1 == s.length) {
                 throw new HttpException("Failed to parse header value");
               }
+              if (preserveBackslash && s[index + 1] != "\"") {
+                sb.write(s[index]);
+              }
               index++;
             } else if (s[index] == "\"") {
               index++;
               break;
             }
             sb.write(s[index]);
             index++;
           }
           return sb.toString();
         } else {
@@ skipping 48 matching lines @@
     if (charset != null) {
       _ensureParameters();
       this._parameters._map["charset"] = charset.toLowerCase();
     }
   }
 
   _ContentType._();
 
   static _ContentType parse(String value) {
     var result = new _ContentType._();
-    result._parse(value, ";");
+    result._parse(value, ";", false);
     int index = result._value.indexOf("/");
     if (index == -1 || index == (result._value.length - 1)) {
       result._primaryType = result._value.trim().toLowerCase();
       result._subType = "";
     } else {
       result._primaryType =
           result._value.substring(0, index).trim().toLowerCase();
       result._subType = result._value.substring(index + 1).trim().toLowerCase();
     }
     return result;
@@ skipping 162 matching lines @@
   void clear() {
     throw new UnsupportedError("Cannot modify an unmodifiable map");
   }
   void forEach(void f(K key, V value)) => _map.forEach(f);
   Iterable<K> get keys => _map.keys;
   Iterable<V> get values => _map.values;
   int get length => _map.length;
   bool get isEmpty => _map.isEmpty;
   bool get isNotEmpty => _map.isNotEmpty;
 }