Navigation.ts doesn't use Google's Alert system

[hannahbergam]

Was able to test locally that using Blockly.dialog.alert(hint) instead of alert(hint) fixes an error with the existing alerts

[microbit-matt-hillsdon]

I agree we should do this but also see #230 (demo) where we're exploring using a different UX for this.

[hannahbergam]

I agree we should do this but also see #230 (demo) where we're exploring using a different UX for this.

Makes sense! I guess I just wanted to make sure that our alert overrides will still be picked up, and the existing Blockly.dialog.alert() checks for those:)