MDL-75365 javascript: Support ES6 in all JS except YUI

.eslintrc

@@ -6,6 +6,10 @@
   ],
   'env': {
     'browser': true,
+    // Enable ES6+ features by default.
+    // See http://eslint.org/docs/user-guide/configuring#specifying-environments
+    // Note: The YUI override must exactly match this when disabling the ES6+ version because those features are not supported by Shifter.
+    'es2020': true,
     'amd': true
   },
   'globals': {
@@ -196,6 +200,10 @@
   overrides: [
     {
       files: ["**/yui/src/**/*.js"],
+      'env': {
+        // Disable ES6+ for YUI files.
+        'es2020': false,
+      },
       // Disable some rules which we can't safely define for YUI rollups.
       rules: {
         'no-undef': 'off',
@@ -221,10 +229,6 @@
     },
     {
       files: ["**/amd/src/*.js", "**/amd/src/**/*.js", "Gruntfile.js", ".grunt/*.js", ".grunt/tasks/*.js", "jsdoc.conf.js"],
-      // We support es6 now. Woot!
-      env: {
-        es6: true
-      },
       // We're using babel transpiling so use their parser
       // for linting.
       parser: '@babel/eslint-parser',
@@ -299,7 +303,6 @@
         ],
       },
       parserOptions: {
-        'ecmaVersion': 9,
         'sourceType': 'module',
         'requireConfigFile': false,
       }