Fix all dartanalyzer errors in dart:sky except the Native ones.

sky/engine/core/events/Event.idl

 /*
  * Copyright (C) 2006, 2007, 2009, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Samuel Weinig <sam.weinig@gmail.com>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Library General Public License for more details.
  *
  * You should have received a copy of the GNU Library General Public License
  * along with this library; see the file COPYING.LIB.  If not, write to
  * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  * Boston, MA 02110-1301, USA.
  */
 
 [
-    Constructor(DOMString type, [Named] optional boolean bubbles, [Named] optional boolean cancelable),
+    // TODO(eseidel): Type is optional to appease dart analyzer:
+    // The class 'Event' does not have a default constructor
+    Constructor([Named] optional DOMString type, [Named] optional boolean bubbles, [Named] optional boolean cancelable),
 ] interface Event {
     // DOM PhaseType
     const unsigned short NONE                = 0;
     const unsigned short CAPTURING_PHASE     = 1;
     const unsigned short AT_TARGET           = 2;
     const unsigned short BUBBLING_PHASE      = 3;
 
     readonly attribute DOMString        type;
     readonly attribute EventTarget      target;
     readonly attribute EventTarget      currentTarget;
     readonly attribute unsigned short   eventPhase;
     [InitializedByEventConstructor] readonly attribute boolean bubbles;
     [InitializedByEventConstructor] readonly attribute boolean cancelable;
     readonly attribute double timeStamp;
 
     void               stopPropagation();
     void               preventDefault();
 
     // DOM Level 3 Additions.
     readonly attribute boolean defaultPrevented;
     void stopImmediatePropagation();
 
     // IE Extensions
     readonly attribute EventTarget srcElement;
     [CallWith=ExecutionContext, ImplementedAs=legacyReturnValue] attribute boolean returnValue;
     attribute boolean cancelBubble;
     readonly attribute NodeList path;
 };