Allow ES2018 as input when running closure compiler

This match the input we accept when running the acorn compiler.

Since we still want to output ES5 by default, also set language
out to ES5 for now.

See #11984

tests/test_other.py

@@ -7760,7 +7760,11 @@ def build():
     self.assertIdentical(normal, tiny)
     self.assertIdentical(normal, huge)
 
-  def test_EM_ASM_ES6(self):
+  @parameterized({
+    '': ([],), # noqa
+    'closure': (['--closure', '1'],), # noqa
+  })
+  def test_EM_ASM_ES6(self, args):
     create_test_file('src.cpp', r'''
 #include <emscripten.h>
 int main() {
@@ -7771,7 +7775,7 @@ def test_EM_ASM_ES6(self):
   });
 }
 ''')
-    self.run_process([EMCC, 'src.cpp', '-O2'])
+    self.run_process([EMCC, 'src.cpp'] + args)
     self.assertContained('hello!', self.run_js('a.out.js'))
 
   def test_check_sourcemapurl(self):

tools/building.py

@@ -1089,8 +1089,10 @@ def add_to_path(dirname):
     outfile = filename + '.cc.js'
     configuration.get_temp_files().note(outfile)
 
-    args = ['--compilation_level', 'ADVANCED_OPTIMIZATIONS' if advanced else 'SIMPLE_OPTIMIZATIONS',
-            '--language_in', 'ECMASCRIPT5']
+    args = ['--compilation_level', 'ADVANCED_OPTIMIZATIONS' if advanced else 'SIMPLE_OPTIMIZATIONS']
+    # Keep in sync with ecmaVersion in tools/acorn-optimizer.js
+    args += ['--language_in', 'ECMASCRIPT_2018']
+
     for e in CLOSURE_EXTERNS:
       args += ['--externs', e]
     args += ['--js_output_file', outfile]