Build tool - Source code transformation tool. Gulp - task runner, used in this context as a build tool for source code.
- https://github.com/Polymer/tools/blob/master/packages/cli/src/build/build.ts & https://github.com/Polymer/tools/tree/master/packages/build/src & https://github.com/Polymer/tools/blob/master/packages/project-config/src/builds.ts#L174