Generalize assert syntax in specification

[eernstg]

With reference to #17682 for background, the language specification should be adjusted such that it uses the grammar rule

assertStatement: assert '(' expression ')' ';'

rather than the current one which uses conditionalExpression. The wording of the paragraph starting with 'The conditional expression e ..' in section 17.17 should be adjusted accordingly.

The language team analysis of this issue is available at https://gist.github.com/eernstg/50a327addd96035d5393a507c0ea2d9a.

[bwilkerson]

Should we also update the specification to reflect the optional message that I believe is already supported by all of the platforms?

[floitschG]

Let's do this independently, but yes.

[lrhn]

Is the optional message supported anywhere? I don't think we ever agreed on a design.

[bwilkerson]

Analyzer supports an optional message (see #24217). According to #24213 it is also supported in dart2js and the formatter (though that issue was never updated to reflect that analyzer supports it). The discussion was around whether to require the second argument to be a String or whether to allow any object and use toString() to convert it to a string.

[lrhn]

... or not convert it at all, and keep it as-is in the AssertionError object.

[munificent]

Closing this because the commit message says it's fixed.

GitHub's "auto-close" looks for certain strings like "Fixes #...", but "fixes issue #..." isn't one of them. :)