Remove codes that require expensive polyfill (#1469)

* Revert "Cleanup domTree.js to re-use code (#1305)"

This reverts commit 9bb48b83f1.

* Cleanup domTree.js to re-use code

* Extract common constructor/methods into functions

* Remove for...of

* Added eslint rule to disallow for...of

* Remove array destructuring

* Added eslint rule to disallow array destructuring

* Add eslint rule to disallow class inheritance

* Remove Object.keys and Object.assign

* Do not polyfill Object.freeze

* Babel: enable loose mode

* Undo disabling `linebreak-style`

* Move `children` initialization out of `initNode`

* Blacklist files for `no-restricted-syntax`

* Revert "Remove array destructuring"

This reverts commit c9d52c2db31c68cca77fea6ad774ee58b0632ff3.

.eslintrc

@@ -70,6 +70,13 @@
         "valid-jsdoc": 0,
         "require-jsdoc": 0
     },
+    "overrides": [{
+        "files": ["katex.js", "src/**/*.js"],
+        "excludedFiles": "unicodeMake.js",
+        "rules": {
+            "no-restricted-syntax": [2, "ForOfStatement", "ClassDeclaration[superClass]", "ClassExpression[superClass]"]
+        }
+    }],
     "env": {
         "es6": true,
         "node": true,