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// |str.substring(0, 1)| is compiled as |str.charAt(0)|.
const strings = [
"",
"a", "b",
"ab", "ba",
];
for (let i = 0; i < 1000; ++i) {
let str = strings[i % strings.length];
assertEq(str.substring(0, 1), str.charAt(0));
}