Add new HttpMultipartFormData, used for parsing a MimeMultipart and extracting either text or binar…

sdk/lib/io/http_headers.dart

Index: sdk/lib/io/http_headers.dart
diff --git a/sdk/lib/io/http_headers.dart b/sdk/lib/io/http_headers.dart
index f6f756bce094cf9b7d5fe6f6ed9d391f17e8f676..598371732c81b9b090e019dd309c90b675b4d1f2 100644
--- a/sdk/lib/io/http_headers.dart
+++ b/sdk/lib/io/http_headers.dart
@@ -525,7 +525,7 @@ class _HeaderValue implements HeaderValue {
             s[index] == parameterSeparator) break;
         index++;
       }
-      return s.substring(start, index).toLowerCase();
+      return s.substring(start, index);
     }
 
     void expect(String expected) {