Specs: clean up event listeners a bit. Drop the pointless 'preventDefault()', since that doesn't really do anything now. Clean up how dispatchEvent() works. Add a 'handled' boolean that listeners can check to see if they should ignore the event.
Specs: clean up event listeners a bit. Drop the pointless
'preventDefault()', since that doesn't really do anything now. Clean
up how dispatchEvent() works. Add a 'handled' boolean that listeners
can check to see if they should ignore the event.

Review URL: https://codereview.chromium.org/752093003

jamesr/flutter_engine,jason-simmons/flutter_engine,krisgiesing/sky_engine,takaaptech/sky_engine,zhangxq5012/sky_engine,krisgiesing/sky_engine,afandria/sky_engine,jamesr/flutter_engine,jason-simmons/sky_engine,jimsimon/sky_engine,mxia/engine,jason-simmons/sky_engine,jason-simmons/sky_engine,flutter/engine,mikejurka/engine,axinging/sky_engine,afandria/sky_engine,mxia/engine,iansf/sky_engine,qiankunshe/sky_engine,Hixie/sky_engine,lyceel/engine,jason-simmons/flutter_engine,TribeMedia/sky_engine,TribeMedia/sky_engine,tvolkert/engine,qiankunshe/sky_engine,mpcomplete/flutter_engine,mdakin/engine,jamesr/sky_engine,mpcomplete/flutter_engine,qiankunshe/sky_engine,jimsimon/sky_engine,devoncarew/sky_engine,abarth/sky_engine,takaaptech/sky_engine,flutter/engine,flutter/engine,iansf/sky_engine,jamesr/flutter_engine,mpcomplete/engine,devoncarew/engine,Hixie/sky_engine,devoncarew/sky_engine,mdakin/engine,jamesr/flutter_engine,lyceel/engine,takaaptech/sky_engine,cdotstout/sky_engine,jason-simmons/sky_engine,chinmaygarde/flutter_engine,tvolkert/engine,takaaptech/sky_engine,mxia/engine,rmacnak-google/engine,flutter/engine,mpcomplete/flutter_engine,krisgiesing/sky_engine,krisgiesing/sky_engine,mikejurka/engine,rmacnak-google/engine,chinmaygarde/sky_engine,zhangxq5012/sky_engine,krisgiesing/sky_engine,aam/engine,afandria/sky_engine,jimsimon/sky_engine,mdakin/engine,jamesr/sky_engine,cdotstout/sky_engine,xunmengfeng/engine,jamesr/flutter_engine,TribeMedia/sky_engine,mxia/engine,tvolkert/engine,Hixie/sky_engine,Hixie/sky_engine,afandria/sky_engine,chinmaygarde/flutter_engine,takaaptech/sky_engine,jamesr/sky_engine,zhangxq5012/sky_engine,xunmengfeng/engine,jamesr/flutter_engine,lyceel/engine,tvolkert/engine,mpcomplete/engine,xunmengfeng/engine,mxia/engine,rmacnak-google/engine,lyceel/engine,TribeMedia/sky_engine,mxia/engine,mikejurka/engine,mxia/engine,chinmaygarde/flutter_engine,jason-simmons/flutter_engine,takaaptech/sky_engine,mxia/engine,devoncarew/engine,jason-simmons/sky_engine,axinging/sky_engine,aam/engine,rmacnak-google/engine,axinging/sky_engine,chinmaygarde/sky_engine,chinmaygarde/flutter_engine,cdotstout/sky_engine,cdotstout/sky_engine,krisgiesing/sky_engine,mpcomplete/engine,chinmaygarde/flutter_engine,devoncarew/engine,xunmengfeng/engine,qiankunshe/sky_engine,jamesr/flutter_engine,devoncarew/sky_engine,abarth/sky_engine,chinmaygarde/sky_engine,qiankunshe/sky_engine,takaaptech/sky_engine,cdotstout/sky_engine,iansf/sky_engine,afandria/sky_engine,TribeMedia/sky_engine,lyceel/engine,axinging/sky_engine,axinging/sky_engine,devoncarew/sky_engine,aam/engine,chinmaygarde/flutter_engine,TribeMedia/sky_engine,iansf/sky_engine,abarth/sky_engine,afandria/sky_engine,jimsimon/sky_engine,jason-simmons/sky_engine,mpcomplete/engine,zhangxq5012/sky_engine,tvolkert/engine,xunmengfeng/engine,abarth/sky_engine,devoncarew/sky_engine,iansf/sky_engine,cdotstout/sky_engine,qiankunshe/sky_engine,takaaptech/sky_engine,jason-simmons/flutter_engine,mpcomplete/engine,jason-simmons/sky_engine,zhangxq5012/sky_engine,afandria/sky_engine,jimsimon/sky_engine,jimsimon/sky_engine,chinmaygarde/sky_engine,afandria/sky_engine,chinmaygarde/sky_engine,jason-simmons/flutter_engine,rmacnak-google/engine,lyceel/engine,axinging/sky_engine,iansf/sky_engine,devoncarew/engine,chinmaygarde/flutter_engine,TribeMedia/sky_engine,zhangxq5012/sky_engine,flutter/engine,mpcomplete/engine,jamesr/sky_engine,iansf/sky_engine,flutter/engine,jamesr/sky_engine,jason-simmons/flutter_engine,Hixie/sky_engine,devoncarew/engine,Hixie/sky_engine,cdotstout/sky_engine,chinmaygarde/sky_engine,lyceel/engine,jimsimon/sky_engine,mikejurka/engine,aam/engine,devoncarew/sky_engine,qiankunshe/sky_engine,mikejurka/engine,qiankunshe/sky_engine,flutter/engine,TribeMedia/sky_engine,jimsimon/sky_engine,mpcomplete/flutter_engine,jimsimon/sky_engine,jason-simmons/flutter_engine,abarth/sky_engine,mdakin/engine,rmacnak-google/engine,mdakin/engine,mikejurka/engine,jamesr/flutter_engine,mikejurka/engine,jason-simmons/flutter_engine,mikejurka/engine,mdakin/engine,mdakin/engine,tvolkert/engine,zhangxq5012/sky_engine,takaaptech/sky_engine,mikejurka/engine,mpcomplete/flutter_engine,zhangxq5012/sky_engine,xunmengfeng/engine,aam/engine,tvolkert/engine,chinmaygarde/sky_engine,flutter/engine,abarth/sky_engine,Hixie/sky_engine,jamesr/sky_engine,aam/engine,axinging/sky_engine,mpcomplete/engine,devoncarew/engine,Hixie/sky_engine,xunmengfeng/engine,aam/engine,jamesr/sky_engine,iansf/sky_engine,abarth/sky_engine,mpcomplete/engine,mdakin/engine,aam/engine,axinging/sky_engine,devoncarew/sky_engine,jamesr/flutter_engine,TribeMedia/sky_engine,devoncarew/engine,axinging/sky_engine,krisgiesing/sky_engine,zhangxq5012/sky_engine,mpcomplete/flutter_engine,rmacnak-google/engine