change dartpad language tour use cases from embedded snippets to a TOC style UI

[devoncarew]

The idea here is to:

• remove embedded dartpads (like in the language tour)
• instead, move to a TOC style full-page UI (similar to try.kotlinlang.org and others)

The two goals here being:

• we think the TOC style would be a superior way to explore and teach the language
• overall, reduce the maintenance cost of dartpad (the embedded version os written in Polymer, which has a higher overall maintenance cost. A TOC UI would hopefully share much of its code with the desktop version of the dartpad UI).

[chalin]

I see two distinct DartPad issues here:

1. Add support for TOC style full-page UI.
   So the goal would be to offer an IDE-like experience with a TOC-style project explorer pane? https://stackblitz.com is another remarkable example (which, as you probably know, is used by the Angular TS team -
   Is there a separate issue to track the evolution of this feature?
2. Deprecate support for embedded DartPads.

Online IDEs like Stackblitz are great! Note that (1), seems to contradict #820: if we're going to offer an IDE-like experience with a TOC-style project explorer pane, it wouldn't make sense to disallow the editing of HTML and CSS files.

Of course, we'll need a transition period between the completion of (1) and (2). /cc @kwalrath

[devoncarew]

https://stackblitz.com looks great! I had not been aware of it.

So the goal would be to offer an IDE-like experience

I would walk that back a bit; this would be a TOC of organized snippets on the left - with a general theme, like learning Dart - and a single editor pane on the right. Selecting a topic would replace the editor's text. We wouldn't try and have multiple editors or editor tabs, or have a sophisticated persistence mechanism.

Agree that 1) and 2) are conflated, but likely to keep DartPad maintainable overall, we'd trade one feature for the other.

[RedBrogdon]

I'm hijacking this bug for the Q1/Q2 project. Based on the plans we've ironed out in the past couple of weeks, the tasks here would go from:

• remove embedded dartpads (like in the language tour)
• instead, move to a TOC style full-page UI (similar to try.kotlinlang.org and others)

to:

• Create a new embedded UI to use dart:html rather than polymer and support some new features (like code verification via unit tests).
• Introduce a TOC-style list of snippets/lessons similar to try.kotlinlang.org (et al) to the full-screen DartPad UI.

These are large goals, so there will be a number of other issues referencing them.

[parlough]

Closing this in favor of #2702 and some other related work Marya and I are exploring. I'll update this comment with a link to that once a tracking issue is available. Thanks!