Fix settings migration. Add lint rule.

.eslintrc.js

@@ -60,6 +60,8 @@ module.exports = {
         'no-trailing-spaces': 'error',
         'object-curly-spacing': ['error', 'always'],
         'space-infix-ops': 'error',
+        'no-unused-expressions': ['error', { allowShortCircuit: true, allowTernary: true }],
+        'no-cond-assign': 'error',
 
         // These rules should eventually be enabled.
         'no-async-promise-executor': 'off',