document ECMAScript quirks (#5501)

closes #5500

README.md

@@ -1429,3 +1429,13 @@ To allow for better optimizations, the compiler makes various assumptions:
   // Actual:   "undefined 42"
   ```
   UglifyJS may modify the input which in turn may suppress those errors.
+- Later versions of JavaScript will throw `SyntaxError` with the following:
+  ```javascript
+  "use strict";
+  console.log(function f() {
+      return f = "PASS";
+  }());
+  // Expected: "PASS"
+  // Actual:   TypeError: invalid assignment to const 'f'
+  ```
+  UglifyJS may modify the input which in turn may suppress those errors.

test/ufuzz/index.js

@@ -2127,7 +2127,9 @@ function errorln(msg) {
 }
 
 function try_beautify(code, toplevel, result, printfn, options) {
-    var beautified = UglifyJS.minify(code, JSON.parse(beautify_options));
+    var o = JSON.parse(beautify_options);
+    if (async && has_await) o.module = true;
+    var beautified = UglifyJS.minify(code, o);
     if (beautified.error) {
         printfn("// !!! beautify failed !!!");
         printfn(beautified.error);