Fix the error message widget

lib/runtime/messages.css

 /** Copyright (c) 2015, 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. **/
 .dev-compiler-messages {
   position: fixed;
   bottom: 0;
   right: 0;
   max-width: 80vw;
   z-index: 10000;
   font-family: sans-serif !important;
 }
 .dev-compiler-messages .message {
   padding: 0.6em;
   background: black;
   color: white;
   border: solid 1px #666;
   border-bottom: 0px;
   text-overflow: ellipsis;
   overflow-x: hidden;
 }
 .dev-compiler-messages .fine {
   color: green;
 }
 .dev-compiler-messages .info {
   color: yellow;
 }
 .dev-compiler-messages .warning {
   color: orange;
 }
-.dev-compiler-messages .severe {
+.dev-compiler-messages .error {
   color: red;
 }
 .dev-compiler-messages div.text {
   white-space: nowrap;
   cursor: pointer;
 }
 .dev-compiler-messages div.text.expanded {
   white-space: normal;
 }
 .dev-compiler-messages div.text a {
@@ skipping 28 matching lines @@
 .dev-compiler-messages .content > div.active {
   display: block;
 }
 
 .dev-compiler-messages .content span.text {
   padding: 0.4em 0.2em 0.2em 2em;
   white-space: pre;
   display: block;
   font-family: monospace !important;
 }