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### Description
<!-- Example: Build failure when loading scripts from CDNs -->

### Versions & Environment
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`polymer --version` will show the version for Polymer CLI
`node --version` will show the version for node
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- Polymer CLI:
- node:
- Operating System:

#### Steps to Reproduce
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Example:

1. Create an application project: `polymer init application`
2. Add script tag to index.html: `<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.2.4/jquery.min.js"></script>`
3. Build: `polymer build`
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#### Expected Results
<!-- Example: No error is throw -->

#### Actual Results
<!-- Example: Error is thrown -->
