Allow ES2018 as input when running closure compiler

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

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 +1089,16 @@ 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']
+    # Tell closure not to do any tranpiling or inject any polyfills.
+    # At some point we may want to look into using this as way to convert to ES5 but
+    # babel is perhaps a better tool for that.
+    args += ['--language_out', 'NO_TRANSPILE']
+    # Tell closure never to inject the 'use strict' directive.
+    args += ['--emit_use_strict=false']
+
     for e in CLOSURE_EXTERNS:
       args += ['--externs', e]
     args += ['--js_output_file', outfile]