Run the following command one by one:

npm install -g vulcanize crisper

Use

• Vulcanize: Crunches all the HTML import files into a single file
• Crisper: Extracts inline js to its own file

Note: Ubuntu users may need to prefix the above command with sudo.

Put all the html imports in your files in a single file elements.html. Don't worry about a file being imported more than once, it'll be crunched down to a single import.

on your elements.html file, run the following commands:

cd PATH/TO/IMPORTFILE/
vulcanize elements.html -o elements.vulc.html --strip-comments --inline-css --inline-js
crisper --source elements.vulc.html --html build.html --js build.js

Vulcanize retrieved source code of all the imports, then replaced imports by their source code.
Crisper took all the js out of the elements.vulc.html file, put it in single build.js file, set a script tag referring the build.js file in the build.html file

• Open HTML minifier

• Open build.html

• Copy all its code

• In the HTML minfier's first textarea, paste the code you copied from build.html

• Click Minify button

• In the second textarea, minified code will appear. Copy that

• Create a build.min.html file and paste all your copied code in it

• Open JSCompress

• Select 2nd tab i.e. Upload JavaScript Files

• Click on Choose Files

• Select build.js file

• Click on Compress button

• Download the file as build.js in the same directory as of build.min.html

Remove all previous imports from those HTmL files from which you copy-pasted the imports. Replace imports with

<link rel="import" href="PATH/TO/IMPORTFILE/build.min.html">