Avoid calling Object.assign before polyfill is installed

[dreamofabear]

Breaks AMP on browsers that don't have Object.assign which appears to be Chrome 44 and earlier among others. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign#Browser_compatibility

Verified locally that this fixes the bug in Chrome 41.

Context: b/112598137

/to @jridgewell @rsimha

Fixes #17505
Fixes #17533
Fixes #17528
Fixes #17547

[rsimha]

Is it feasible to write a unit test (or an error message) for this, such that it would work on latest Chrome (by checking that a polyfilled function is never called before it is installed?)

[dreamofabear]

Was just thinking about this. We know to import polyfills.js first in entry points, but this bug was due to order of execution in the file itself. To catch that, we can test by importing polyfills.js in an environment that's missing all of the native APIs and making sure there are no runtime errors.

[rsimha]

The offending code was introduced #17205 In 2246723, which was released last week. I wonder why things only broke one week later.

Edit: It turns out that the release went to canary 6 days ago, but hit prod yesterday. This now make sense.