Revert "Removed Google Closure compiler because it's failing"

This reverts commit 4594e0e.
We are unable to make use of the other parsers, so unminifying
the python parser does not benefit us. It slows down load speeds.

webpack.config.js

@@ -65,6 +65,24 @@ const prepare_lang = async function(filename) {
         // Tell webpack that fs will be external so it doesn't complain.
         'externals': ['fs'],
 
+        'plugins': [
+            // Run the Google Closure Compiler if env.optimize is set
+            optimize ? new ClosureCompilerPlugin({
+                'compiler': {
+                    'language_in': 'ECMASCRIPT6',
+                    'language_out': 'ECMASCRIPT5',
+                    'compilation_level': 'SIMPLE_OPTIMIZATIONS',
+                    /*
+                    There is a higher compilation level, ADVANCED_OPTIMIZATIONS.
+                    Unfortunately, it requires property access by string or key to be consistent.
+                    For example, obj.abc = 123; alert(obj['abc']); doesn't work.
+                    Antlr does not follow this principle, so as far as I can see, we won't be able to use this level.
+                    */
+                },
+                'concurrency': 10,
+            }) : undefined,
+        ].filter(Boolean),
+
         'output': {
             // The output filename
             'filename': lang_name + (optimize ? '.min.js' : '.js'),