Tries to build .d.ts files #318
Description
In the static build Zwitterion tries to build .d.ts files as if they are JS files and totally fails when there is no indication that it should fail.
Example: having a dependency on @polymer/iron-a11y-announcer version 3.0.2. The file node_modules/@polymer/iron-a11y-announcer/iron-a11y-announcer.d.ts breaks the build.
This worked on 0.29.0 but doesn't now on 0.45.0