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The original error is unrelated to the "alternate error".

While the build task runs gulp, npm run prioritizes the locally-installed gulp (at node_modules/.bin/gulp) before the system-installed gulp. Running gulp yourself (without npm run) would invoke the globally-installed gulp, which may result in an error if it's incompatible with your project (e.g., Gulp 3 binary with Gulp 4 API in your scripts, which appears to be the case). You could either install Gulp 4 so that you can run gulp yourself, or continue using npm run build.

To troubleshoot the original error, I recommend starting from the Polycast's original source (if you haven't already) to determine what the difference could be.

If you prefer to stick with your current track, I suggest a few things:

• Verify the paths in your HTML imports, as a path to a non-existent file would cause a silent error (polymer-build issue 88). It might be helpful to run polymer build -v (verbose build).
• Add buildStream.on('error', (err) => console.log(err)) after let buildStream = ... in case any unsuppressed error events crop up in that stream.