Remove limits of ES6 features in standard library code

When transpiling to ES5 use closure with `SIMPLE_OPTIMIZATIONS` rather
then `WHITESPACE_ONLY`.  This means that polyfills cant be included
as needed and removes the limits on what ES6 features we can use.

The downside of this is that is slows down builds for ES5 users but
this seems like a reasonable tradeoff.

Fixes: emscripten-core#11984

ChangeLog.md

@@ -27,6 +27,10 @@ See docs/process.md for more on how version tagging works.
   `warning: treating 'c' input as 'c++' when in C++ mode`.  This also means that
   the `DEFAULT_TO_CXX` setting now only applies when linking and not when
   compiling. (#20712)
+- JavaScript library code can now use the full range ES6 features and we rely
+  on closure compiler to transpile for ES5 when targetting older browsers.
+  For those that want to would rather perform transpilation seperately outside
+  of emscripten you can use the `-sPOLYFILL=0` setting.  (#20700)
 
 3.1.49 - 11/14/23
 -----------------

emcc.py

@@ -2318,7 +2318,7 @@ def phase_linker_setup(options, state, newargs):
 
   setup_environment_settings()
 
-  if options.use_closure_compiler != 0:
+  if options.use_closure_compiler != 0 and settings.POLYFILL:
     # Emscripten requires certain ES6 constructs by default in library code
     # - https://caniuse.com/let              : EDGE:12 FF:44 CHROME:49 SAFARI:11
     # - https://caniuse.com/const            : EDGE:12 FF:36 CHROME:49 SAFARI:11

test/test_other.py

@@ -12990,7 +12990,7 @@ def test_hello_function(self):
   })
   @crossplatform
   def test_es5_transpile(self, args):
-    self.emcc_args += args
+    self.emcc_args += ['-Wno-transpile'] + args
 
     # Create a library file that uses the following ES6 features
     # - let/const
@@ -13006,6 +13006,11 @@ def test_es5_transpile(self, args):
         let obj = Object.assign({}, {prop:1});
         err('prop: ' + obj.prop);
 
+        // for .. of
+        for (var elem of [42, 43]) {
+          err('array elem: ' + elem);
+        }
+
         // arrow funcs + const
         const bar = () => 2;
         err('bar: ' + bar());
@@ -13015,14 +13020,14 @@ def test_es5_transpile(self, args):
         var obj2 = {
           [key]: 42,
         };
-        err('value: ' + obj2[key]);
+        err('computed prop: ' + obj2[key]);
 
         // Method syntax
         var obj3 = {
           myMethod() { return 43 },
         };
         global['foo'] = obj3;
-        err('value2: ' + obj3.myMethod());
+        err('myMethod: ' + obj3.myMethod());
 
         // Nullish coalescing
         var definitely = global['maybe'] ?? {};
@@ -13042,6 +13047,15 @@ def test_es5_transpile(self, args):
       }
     });
     ''')
+    expected = '''\
+prop: 1
+array elem: 42
+array elem: 43
+bar: 2
+computed prop: 42
+myMethod: 43
+'''
+
     create_file('test.c', 'extern void foo(); int main() { foo(); }')
     self.emcc_args += ['--js-library', 'es6_library.js']
     self.uses_es6 = True
@@ -13073,26 +13087,26 @@ def check_for_es6(filename, expect):
     # Check that under normal circumstances none of these features get
     # removed / transpiled.
     print('base case')
-    self.do_runf('test.c', 'prop: 1\nbar: 2\n')
+    self.do_runf('test.c', expected)
     check_for_es6('test.js', True)
 
     # If we select and older browser than closure will kick in by default
     # to transpile.
     print('with old browser')
     self.emcc_args.remove('-Werror')
     self.set_setting('MIN_CHROME_VERSION', '10')
-    self.do_runf('test.c', 'prop: 1\nbar: 2\n', output_basename='test_old')
+    self.do_runf('test.c', expected, output_basename='test_old')
     check_for_es6('test_old.js', False)
 
     # If we add `--closure=0` that transpiler (closure) is not run at all
     print('with old browser + --closure=0')
-    self.do_runf('test.c', 'prop: 1\nbar: 2\n', emcc_args=['--closure=0'], output_basename='test_no_closure')
+    self.do_runf('test.c', expected, emcc_args=['--closure=0'], output_basename='test_no_closure')
     check_for_es6('test_no_closure.js', True)
 
     # If we use `--closure=1` closure will run in full optimization mode
     # and also transpile to ES5
     print('with old browser + --closure=1')
-    self.do_runf('test.c', 'prop: 1\nbar: 2\n', emcc_args=['--closure=1'], output_basename='test_closure')
+    self.do_runf('test.c', expected, emcc_args=['--closure=1'], output_basename='test_closure')
     check_for_es6('test_closure.js', False)
 
   def test_gmtime_noleak(self):

tools/building.py

@@ -513,7 +513,8 @@ def closure_transpile(filename):
   user_args = []
   closure_cmd, env = get_closure_compiler_and_env(user_args)
   closure_cmd += ['--language_out', 'ES5']
-  closure_cmd += ['--compilation_level', 'WHITESPACE_ONLY']
+  closure_cmd += ['--compilation_level', 'SIMPLE_OPTIMIZATIONS']
+  closure_cmd += ['--formatting', 'PRETTY_PRINT']
   return run_closure_cmd(closure_cmd, filename, env)