RenderObjectWrapper cleanup and new asserts.
RenderObjectWrapper cleanup and new asserts.

- Clarify comment about RenderObjectWrapper.
- Assert that we never sync a RenderObjectWrapper with an instance of a different type.
- Assert that MultiChildRenderObjectWrapper subclasses always have multi-child RenderObjects.
- Assert that renderObject doesn't change identity when syncing.
- Make searchForOldNode() return void since the return value is ignored.
- Remove code that handled renderObject changing during sync.

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