Fix polymer lint input to exclude node_modules

node_modules should already be excluded by default, but it doesn't seem to be the case. After an update to a transitive dependency Popper.js to 1.16.0, we started to get the following erros locally and on CI:

node_modules/popper.js/src/index.js(NaN,NaN) error [parse-error] - This experimental syntax requires enabling the parser plugin: 'classProperties' (111:17)