Roll to branched dart sdk with two more hotfixes for flutter 1.5.4.

[aam]

Cherry-picks for flutter/flutter#29007 and VERSION file bump.

[aam]

cc @athomas

[tvolkert]

Looks like this needs a license signature update

[aam]

Looks like this needs a license signature update

Done

[tvolkert]

The Dart version bump to 2.3.0 could be problematic because it'll mean that Flutter v1.5.4-hotfix.[n] will report a newer Dart version than v1.5.5-v1.y.z (where y and z are probably already bounded, but I didn't take the time to look them up). If any of those versions ever make it to stable, it'll potentially mess with the SDK constraints of pub packages.

[aam]

The Dart version bump to 2.3.0 could be problematic because it'll mean that Flutter v1.5.4-hotfix.[n] will report a newer Dart version than v1.5.5-v1.y.z

VERSION update should not change version dart sdk that flutter reports(has been reporting). dart sdk built in flutter doesn't use VERSION file - instead it looks up value from the tag.

[tvolkert]

Gotcha - what's the significance of the VERSION file bump?

[aam]

Gotcha - what's the significance of the VERSION file bump?

Hm, not sure... @Hixie requested it, need to check with him.