Fix lints: unnecessary_brace_in_string_interp

pkg/analysis_server/tool/spec/from_html.dart

 // Copyright (c) 2014, 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.
 
 /**
  * Code for reading an HTML API description.
  */
 library from.html;
 
 import 'dart:io';
@@ skipping 89 matching lines @@
     if (!requiredAttributes.contains(name) &&
         !optionalAttributes.contains(name)) {
       throw new Exception(
           '$context: Unexpected attribute in ${element.localName}: $name');
     }
     attributesFound.add(name);
   });
   for (String expectedAttribute in requiredAttributes) {
     if (!attributesFound.contains(expectedAttribute)) {
       throw new Exception(
-          '$context: ${element.localName} must contain attribute ${expectedAttribute}');
+          '$context: ${element.localName} must contain attribute $expectedAttribute');
     }
   }
 }
 
 /**
  * Check that the given [element] has the given [expectedName].
  */
 void checkName(dom.Element element, String expectedName, [String context]) {
   if (element.localName != expectedName) {
     if (context == null) {
@@ skipping 424 matching lines @@
       TypeDefinition typeDefinition = typeDefinitionFromHtml(child);
       types[typeDefinition.name] = typeDefinition;
     }
   });
   return new Types(types, html);
 }
 
 typedef void ElementProcessor(dom.Element element);
 
 typedef void TextProcessor(dom.Text text);