Enable new ESLint-recommended rules in ESLint config

Refs http://eslint.org/docs/user-guide/migrating-to-2.0.0#new-rules-in-eslintrecommended

js/.eslintrc.json

@@ -38,6 +38,7 @@
         "no-negated-in-lhs": 2,
         "no-obj-calls": 2,
         "no-regex-spaces": 2,
+        "no-self-assign": 2,
         "no-sparse-arrays": 2,
         "no-unexpected-multiline": 2,
         "no-unreachable": 2,
@@ -173,6 +174,7 @@
         "no-class-assign": 2,
         "no-const-assign": 2,
         "no-dupe-class-members": 2,
+        "no-new-symbol": 2,
         "no-this-before-super": 2,
         "no-var": 2,
         "object-shorthand": 2,