_es5Concatenation(args) {
		const str = args
			.map(arg => (typeof arg === "string" ? JSON.stringify(arg) : arg.expr))
			.join(" + ");

		// when the first two args are expression, we need to prepend "" + to force string
		// concatenation instead of number addition.
		return typeof args[0] !== "string" && typeof args[1] !== "string"
			? `"" + ${str}`
			: str;
	}
@param {Array<string|{expr: string}>} args args (len >= 2)
@returns {string} result expression
@private