Remove another use of deprecated mirror API from dart:html.

tools/dom/src/native_DOMImplementation.dart

 // Copyright (c) 2012, 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 html;
 
 class _ConsoleVariables {
   Map<String, Object> _data = new Map<String, Object>();
 
   /**
@@ skipping 391 matching lines @@
   }
 
   static void register(Document document, String tag, Type type,
       String extendsTagName) {
     // TODO(vsm): Move these checks into native code.
     ClassMirror cls = reflectClass(type);
     if (_isBuiltinType(cls)) {
       throw new UnsupportedError("Invalid custom element from $libName.");
     }
     var className = MirrorSystem.getName(cls.simpleName);
-    var createdConstructor = cls.constructors[new Symbol('$className.created')];
+    var createdConstructor = cls.declarations[new Symbol('$className.created')];
     if (createdConstructor == null ||
         createdConstructor is! MethodMirror ||
         !createdConstructor.isConstructor) {
       throw new UnsupportedError(
           'Class is missing constructor $className.created');
     }
 
     if (createdConstructor.parameters.length > 0) {
       throw new UnsupportedError(
           'Constructor $className.created must take zero arguments');
@@ skipping 186 matching lines @@
   _scheduleImmediateHelper._schedule(callback);
 };
 
 get _pureIsolateScheduleImmediateClosure => ((void callback()) =>
   throw new UnimplementedError("scheduleMicrotask in background isolates "
                                "are not supported in the browser"));
 
 void _initializeCustomElement(Element e) {
   _Utils.initializeCustomElement(e);
 }