Fix canvas getContext signature in lib/lib.dom.d.ts

[bbenezech]

According to
https://developer.mozilla.org/en/docs/Web/API/HTMLCanvasElement/getConte
xt, it only returns null if the context identifier is not supported
(i.e. not “2d” | "webgl" | "experimental-webgl”, when new non-standard
renderers are omitted).

It used to be that way, but was changed part of this (huge) commit, 1c9df84
I cannot find the originating commit in the PR: #9697 but it must be somewhere (my git skills are average).

This fix might make some people happy: this error generates tons of
useless and annoying non-null ! type casts when working with canvases.

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[msftclas]

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Microsoft Pull Request Bot

[mhegazy]

Thanks for your PR. these files are not manually edited, they are auto-generatd from a script in . You can find more information about contributing lib.d.ts fixes at https://github.com/Microsoft/TSJS-lib-generator.

Changes should be done in https://github.com/Microsoft/TSJS-lib-generator instead.

Please find more information at: https://github.com/Microsoft/TypeScript/blob/master/CONTRIBUTING.md#contributing-libdts-fixes.

For future references, please file an issue first to track changes.