Dev mode error for missing loop value

[Rich-Harris]

This results in an unhelpful error — 'Cannot read property 'length' of undefined':

{{#each missing as thing}}
  <p>{{this}}</p>
{{/each}}

In dev mode, we could check that missing is an array-like object, and give a less cryptic error if not.

[Rich-Harris]

Just had a thought. One of the reasons this error is a bit unhelpful is that it doesn't point back to the source code. So whereas in this situation if foo is undefined, the sourcemap shows us where the problem is...

...if foo is defined but foo.bar isn't, it doesn't:

I think we could fix this. If here, and in every other place we access .length, we instead overwrote the each with length and accessed .[✂${a}-${b}✂], where a and b are the start and end positions of the each, then the failed property access would point to the start of each, and clicking on the stack trace in devtools would point you to the offending block.

We should still have the dev mode warning, but this would be a very handy way to get most of the benefits without dev mode.

[Rich-Harris]

It works!

Opening a PR now