Fix Travis build on Node 8. (#233)
Fix Travis build on Node 8. (#233)

Error names now include their new error code: https://nodejs.org/en/blog/release/v8.0.0/#static-error-codes

Build log: https://travis-ci.org/Polymer/polymer-build/jobs/237741989
Polymer/tools,Polymer/tools,Polymer/tools,Polymer/tools