docs(guide): add dart version of structural directives

[adaojunior]

No description provided.

[adaojunior]

I messed up in the other pull request

[kwalrath]

It looks like you haven't incorporated all the comments from #696, so I'll wait for another commit before taking a final look.

[adaojunior]

@kwalrath I removed the unused file and changed to relative imports.

[zoechi]

Any feedback on this string interpol style yet?
The Dart style guide discourages {} here

[adaojunior]

@zoechi Aparently they're only necessary when it is a expression or you're calling a method in a object. I'll remove them. Thanks

[kwalrath]

@sanfordredlich could you please do a technical review? I'm especially curious about the _tick() => new Future(() {}) code in heavy_loader_component.dart. (It's derived from the TS code, but using a Timer resulted in more complicated code, so I suggested new Future() as a way of kicking the event loop.)

[zoechi]

Is "JavaScript cycle" a good explanation here?

[kwalrath]

Good question. How about "browser event loop"? We should perhaps make this change to the JS/TS samples, too.

[zoechi]

Sounds good and should fly in JS land as well

[adaojunior]

done, also sent a PR to change it in the TS example.
#710

[kwalrath]

I'm going to merge this PR, so we can point to this code from a placeholder page. We'll do a final technical review before I write & publish the real page.