🚨 Error: Unexpected end of JSON input

Details
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**Error report:** [link](http://go/ampe/CP-6k7GLrJe77gE)
**First seen:** Apr 12, 2020
**Frequency:** ~ 667/day

Stacktrace
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Error: Unexpected end of JSON input

    at JSON (src/json.js:112:37)

    at buildCallback (src/custom-element.js:477:18)

    at src/custom-element.js:474:38

    at build (src/service/resource.js:336:11)

    at resource (src/service/resources-impl.js:598:20)

    at buildResourceUnsafe_ (src/service/resources-impl.js:533:6)

    at buildOrScheduleBuildForResource_ (src/service/resources-impl.js:650:4)

    at upgraded (src/custom-element.js:383:6)

Notes
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`@rsimha` modified `src/json.js:112-112` in #21212 (May 16, 2019)
`@rsimha` modified `src/service/resource.js:336-338` in #21212 (May 16, 2019)
`@mkhatib` modified `src/service/resources-impl.js:650-650` in #4476 (Aug 12, 2016)
`@jridgewell` modified `src/custom-element.js:383-383` in #8229 (Mar 28, 2017)

**Seen in:**
- 07-30 Stable (2351+1)
- 08-15 Experimental (0009+1)

**Possible assignees:** `@jridgewell`, `@mkhatib`

/cc @ampproject/release-on-duty