firebase_storage dependency issue

[richardcatlin]

Because firebase_storage_platform_interface >=1.0.4 depends on firebase_core ^0.7.0 and firebase_storage depends on firebase_core >=0.8.0-1.0.nullsafety.1 <0.8.0-2.0.nullsafety.0, firebase_storage_platform_interface >=1.0.4 is forbidden.
So, because firebase_storage depends on firebase_storage_platform_interface ^1.0.4, version solving failed.