Find bugs in JavaScript programs.

E0043: unexpected '\' in identifier

Identifiers (variable names, etc.) can contain Unicode escape sequences (\u{69} for example). It is an error for an identifier to have a backslash which is not part of a Unicode escape sequence:

function identity\(x) { return x; }

let bird\U3600 = new ChirpSound();

To fix this error, remove the backslash, or complete the Unicode escape sequence:

function identity(x) { return x; }

let bird\u3600 = new ChirpSound();

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