Enable Dart compressed pointers for 64-bit mobile targets (#27975)
Enable Dart compressed pointers for 64-bit mobile targets (#27975)

* Enable Dart compressed pointers for 64-bit mobile targets

* Fix.

* Address code review comments.
jason-simmons/flutter_engine,aam/engine,Hixie/sky_engine,Hixie/sky_engine,aam/engine,chinmaygarde/flutter_engine,devoncarew/engine,flutter/engine,rmacnak-google/engine,aam/engine,chinmaygarde/flutter_engine,rmacnak-google/engine,Hixie/sky_engine,flutter/engine,flutter/engine,aam/engine,jason-simmons/flutter_engine,jason-simmons/flutter_engine,rmacnak-google/engine,rmacnak-google/engine,flutter/engine,flutter/engine,flutter/engine,devoncarew/engine,chinmaygarde/flutter_engine,devoncarew/engine,rmacnak-google/engine,Hixie/sky_engine,chinmaygarde/flutter_engine,Hixie/sky_engine,aam/engine,devoncarew/engine,Hixie/sky_engine,jason-simmons/flutter_engine,flutter/engine,jason-simmons/flutter_engine,chinmaygarde/flutter_engine,Hixie/sky_engine,rmacnak-google/engine,rmacnak-google/engine,chinmaygarde/flutter_engine,jason-simmons/flutter_engine,flutter/engine,chinmaygarde/flutter_engine,devoncarew/engine,aam/engine,aam/engine,jason-simmons/flutter_engine,devoncarew/engine,aam/engine,Hixie/sky_engine,devoncarew/engine,jason-simmons/flutter_engine