tools: polymer-build breaks if the project path has spaces

Description

polymer-build breaks if the project path has spaces.

Versions & Environment

• polymer-build: v0.6.0-alpha.3
• node: v6.0.0
• Operating System: macOS Sierra v10.12.2

Steps to Reproduce

Using the last Polycast example: https://github.com/Polymer/polycasts/tree/master/ep60-firebase-build

Download the project in a path with spaces. Example: ~/folder name/project-name

npm install
bower install
npm run build

Expected Results

The build work.

Actual Results

The build doesn’t work.

If I rename the ~/folder name/project-name to ~/folder-name/project-name it works.