Bump Acorn JS version to ES2018 (#11435)

We set the version to 6 (2015) when we added Acorn, probably to
keep it as close as possible to the old pre-Acorn parser. But the
default in our version is 9 (2018) so we may as well switch to that,
and that fixes #11431 (8 - 2017 - would have been enough for
that specific bug).

tests/test_other.py

@@ -8628,12 +8628,12 @@ def build():
     self.assertIdentical(normal, huge)
 
   def test_EM_ASM_ES6(self):
-    # check we show a proper understandable error for JS parse problems
     create_test_file('src.cpp', r'''
 #include <emscripten.h>
 int main() {
   EM_ASM({
-    var x = (a, b) => 5; // valid ES6!
+    var x = (a, b) => 5; // valid ES6
+    async function y() {} // valid ES2017
     out('hello!');
   });
 }

tools/acorn-optimizer.js

@@ -1299,7 +1299,7 @@ if (extraInfoStart > 0) {
 // Collect all JS code comments to this array so that we can retain them in the outputted code
 // if --closureFriendly was requested.
 var sourceComments = [];
-var ast = acorn.parse(input, { ecmaVersion: 6, preserveParens: closureFriendly, onComment: closureFriendly ? sourceComments : undefined });
+var ast = acorn.parse(input, { ecmaVersion: 2018, preserveParens: closureFriendly, onComment: closureFriendly ? sourceComments : undefined });
 
 var minifyWhitespace = false;
 var noPrint = false;
@@ -1337,4 +1337,3 @@ if (!noPrint) {
   });
   print(output);
 }
-