Pass --enable_mirrors=false when building script snapshots for the debug mode. (#12472)
Pass --enable_mirrors=false when building script snapshots for the debug mode. (#12472)

The initial loading happens on the host, which was building a script snapshot and allowing imports of dart:mirrors. Hot reload happens on the device, which then notices the imports and issues a compile-time error. This change causes programs with imports of dart:mirrors to be rejected during the initial load.

Fixes https://github.com/flutter/flutter/issues/12440
tvolkert/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,tvolkert/flutter,tvolkert/flutter,cbracken/flutter,cbracken/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,Hixie/flutter,Hixie/flutter,flutter/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,tvolkert/flutter,tvolkert/flutter,flutter/flutter,tvolkert/flutter,cbracken/flutter,cbracken/flutter,Hixie/flutter,tvolkert/flutter,tvolkert/flutter,cbracken/flutter,flutter/flutter,Hixie/flutter,tvolkert/flutter,cbracken/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,Hixie/flutter,flutter/flutter,flutter/flutter