fix one time mustache parsing

[jakemac53]

This fixes one time bindings parsing when using list or map accessors so a trailing space is no longer required:

[[foo['bar']]] now works where you used to have to do [[foo['bar'] ]]

[justinfagnani]

I commented on the Dart version of this CL:

I wonder if we should track balancing of the terminators. A case like [[ foo[bar[0]] ]] looks like it'll terminate early. Same with {{ foo({'x': {'y': 0}}) }}. The requirement of binging delegate would be that 1) {} and [] must
be balanced, and probably also 2) Non-significant terminators chars (like in
strings) are escaped with \

[justinfagnani]

@rafaelw

[jmesserly]

Hmmm. Personally not really a fan of trying to track "balanced" terminators, since we know nothing about the grammar of the expression. The workaround seems fine to me -- just add a space. Not sure this is worth the code size tradeoff. (edit: where "this" means "tracking balanced terminators", not this pull req, which I'm ambivalent about.)

[justinfagnani]

Without tracking balanced tokens I think you have to give up on this PR, since it'll terminate valid expressions early.

[jakemac53]

The previous logic also was terminating valid expressions early, this just handles a common edge case that the previous logic didn't cover with very little extra overhead.

In terms of balancing the terminators I have an implementation on the dart side now that assumes escaping of terminators inside of strings, and otherwise allows you to escape terminators outside of strings (forcing escaping inside of strings seems extreme, and I think its a fair assumption that quoted things are strings). That adds around 50 relatively short lines right now. We could remove the escaping terminators logic and assume a sane language with balanced brackets to get rid of around 15-20 lines of that code.

If we do go with the balancing terminators route, its definitely a breaking change as well so that should be considered.

For reference the current dart implementation is here: https://codereview.chromium.org/712333006/patch/20001/30002