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Several years ago, before all major browsers supported source maps, we felt it was important to provide line number information in generated files using end-of-line comments like
Adding all these comments was always slower than leaving the code unmodified, and recently they have begun interacting badly with certain newer ECMAScript syntax, such as multi-line template strings (#9160).
Since source maps are well supported in most browsers that developers are likely to be using for development, and the line number comments are now causing substantive problems beyond the performance cost, I think it's time we stopped using them once and for all.