The `String` constructor has a special case for `Symbol` conversions:
https://tc39.es/ecma262/#sec-string-constructor-string-value

    a. If NewTarget is undefined and value is a Symbol, return
       SymbolDescriptiveString(value).
    b. Let s be ? ToString(value).

Therefore, the only difference between `String(value)` and `value + ''`
is that the latter will throw for symbols.

Terser uses this to advantage by replacing invocations of `String(...)`
with `... + ''`. Terser, to its credit, has this behavior gated behind
the `unsafe` flag with the following documentation: "It enables some
transformations that might break code logic in certain contrived cases,
but should be fine for most code." For this particular code it is *not*
fine though.

Under WebPack, Terser is typically run per chunk rather than per file.
It is difficult or impossible to disable these optimizations for only
core-js, only for the entire bundle. This optimization causes the Symbol
constructor detection pathway to fail and fallback to string keys. After
everything is said and done we end up with a confusing error
message of "[object Generator] is not iterable".

The fix is simple, we simply bailout of this optimization in this
critical path.