add paper-layout-classes

[naxo8628]

With the import '@polymer/iron-flex-layout/iron-flex-layout.js'; i got this error then import the paper-dialog-scrollable:

GET http://localhost:8081/node_modules/@polymer/paper-dialog-scrollable/node_modules/@polymer/iron-flex-layout/iron-flex-layout.js 404 (Not Found)

Is not generating the path correctly, instead with iron-flex-layout-classes.js work fine

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