const become var and let when using --bundle

[stackinspector]

> echo 'const a = document.getElementById("a"); doSomethingWith(a); (() => { const b = document.getElementById("b"); doSomethingWith(b); })();' | esbuild --minify-whitespace --minify-syntax --format=iife --target=esnext --bundle
(()=>{var a=document.getElementById("a");doSomethingWith(a);(()=>{let b=document.getElementById("b");doSomethingWith(b)})();})();
> echo 'const a = document.getElementById("a"); doSomethingWith(a); (() => { const b = document.getElementById("b"); doSomethingWith(b); })();' | esbuild --minify-whitespace --minify-syntax --format=iife --target=esnext         
(()=>{const a=document.getElementById("a");doSomethingWith(a);(()=>{const b=document.getElementById("b");doSomethingWith(b)})();})();

[stackinspector]

Seems duplicate with #1301.