Change imports to clean up irritating warning

sdk/lib/web_sql/dart2js/web_sql_dart2js.dart

 /**
  * An API for storing data in the browser that can be queried with SQL.
  *
  * **Caution:** this specification is no longer actively maintained by the Web
  * Applications Working Group and may be removed at any time.
  * See [the W3C Web SQL Database specification](http://www.w3.org/TR/webdatabase/)
  * for more information.
  *
  * The [dart:indexed_db] APIs is a recommended alternatives.
  */
 library dart.dom.web_sql;
 
 import 'dart:async';
 import 'dart:collection';
-import 'dart:_internal';
+import 'dart:_internal' show FixedLengthListMixin;
 import 'dart:html';
 import 'dart:html_common';
 import 'dart:_foreign_helper' show JS;
 import 'dart:_interceptors' show Interceptor;
 // DO NOT EDIT - unless you are editing documentation as per:
 // https://code.google.com/p/dart/wiki/ContributingHTMLDocumentation
 // Auto-generated dart:audio library.
 
 import 'dart:_js_helper'
     show
@@ skipping 278 matching lines @@
     throw new UnsupportedError("Not supported");
   }
 
   @DomName('SQLTransaction.executeSql')
   @DocsEditable()
   void executeSql(String sqlStatement,
       [List arguments,
       SqlStatementCallback callback,
       SqlStatementErrorCallback errorCallback]) native;
 }