chore(angular.toJson): use charAt instead of regexp

[just-boris]

Regular Expressions is an expensive operation and this case can be simplified. Here's a comparison: http://jsperf.com/check-first-char
Using charAt is better than using brackets, explanation of this can be found here http://stackoverflow.com/questions/5943726/string-charatx-or-stringx/5943760#5943760

[mary-poppins]

Thanks for the PR!

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Thanks again for your help!

[just-boris]

@mary-poppins I've signed, my name is Boris Serdyuk

[just-boris]

Some browsers sometimes put into as key argument not a string, because there is need additional check.
Here is broken travis build without it: https://travis-ci.org/angular/angular.js/builds/11624597

[petebacondarwin]

Doesn't this mean that toJSON is not ignoring keys starting in $ for IE8 now?

[petebacondarwin]

Can you provide a unit test if there isn't one for IE.
Also I think this counts as a refactor not a chore.

[just-boris]

@petebacondarwin there are already has tests for these keys, isn't it?https://github.com/angular/angular.js/blob/master/test/AngularSpec.js#L982

[petebacondarwin]

Oh, OK. So the check for charAt is in case the key is a number rather than a string?

[just-boris]

@petebacondarwin yes. I've changed these check for more effective. Here's comparison: http://jsperf.com/check-first-char/4
Moreover, I can't reproduce case when key is number on my IE8, it always put string, unlike the travis build

[petebacondarwin]

LGTM