Add Open in IDX feature

[johnpryan]

This adds an API endpoint and UI component to open the current snippet in IDX under the new "Continue in" dropdown.

closes #2984

[devoncarew]

For the UI:

• consider move the "open external" icon from the 'Install SDK' menu item to the 'continue in' menu; this would parallel the 'new' and 'samples' menus
• as the IDX and Install SDK menu items take you somewhere, consider ending them in ellipses (…)

[devoncarew]

I wonder if the 'Install SDK' menu item should move from the continue in menu to the tripple dot overflow menu? It's a different type of action than the IDX one; it doesn't use any of the content of the current snippet.

fwiw, I do like having a 'continue in' menu to hold the IDX item

[devoncarew]

Some comments about the menu presentation, and about how we're forming the http POST.

[johnpryan]

consider move the "open external" icon from the 'Install SDK' menu item to the 'continue in' menu; this would parallel the 'new' and 'samples' menus

This indicates that pressing the button will take you somewhere (at least that's how I interpret it). When I add this, it looks odd to me:

as the IDX and Install SDK menu items take you somewhere, consider ending them in ellipses

I'm not familiar with this pattern, to me it would be weird to have two sets of elipses, "Open in... -> IDX... Install SDK..."

I wonder if the 'Install SDK' menu item should move from the continue in menu to the tripple dot overflow menu? It's a different type of action than the IDX one; it doesn't use any of the content of the current snippet.

Right now we struggle with maintaining so many menu buttons. (For the mobile layout we need hide this button, for example). I'd rather group these together. If there's a dropdown with only one option, that doesn't really add much value to the UI, since it could have just been a button in the first place.

[johnpryan]

Discussed offline, final result:

[romannurik]

Awesome that this is live!! @johnpryan one quick note, I noticed the "Flame game" sample doesn't work as-is because the pubspec.yaml (with the flame package) isn't being sent along... let me know if that's challenging to send across.

[johnpryan]

Thanks for finding that @romannurik, we should be able to fix this. I filed an issue to track: #3005