🚨 Error: Cannot read property 'toString' of undefined

Details
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**Error report:** [link](http://go/ampe/CMfL58qJw-qbBg)
**First seen:** May 9, 2018
**Frequency:** ~ 351/day

Stacktrace
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Error: Cannot read property 'toString' of undefined

    at toString (src/polyfills/custom-elements.js:99:30)

Notes
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`@jridgewell` modified `src/polyfills/custom-elements.js:90-102` in [#17205](https://github.com/ampproject/amphtml/pull/17205) (Aug 3, 2018)

**Seen in:**
- 03-06 Experiment-A (0631+2)
- 03-12 Nightly (2145)

/cc @ampproject/release-on-duty

Start by reading src/polyfills/custom-elements.js around lines 90-102 and the stack trace at line 99. Review pull request #17205, which modified this area, and compare the reported error with the production error report. Done means determining whether that change addresses the undefined toString failure.

Written by the indexing model from the issue text.