Fix getProperties invocation passing length for all objects.

[biggs0125]

The vm_service Instance spec specifies that the length property specifies that length should be the number of non-static fields for plain objects. In order to align with the VM we've updated the DDC debugger API to include this value of length on plain objects.

However, getProperties is assuming that if a length is provided then it should do a range check with the provided offset and count. This was causing the getProperties code to exit early with no value.

To fix this we only pass the length to getProperties if the kind of the object being inspected is not a plain object. Plain objects should never need a range check.

Fixes #2479.

[biggs0125]

It's worth noting that prior to the DDC change that caused this regression, this same bad path could've been reached by having a user-defined class with a length getter that returned a non-zero integer. The same test would've passed an offset and ended up in the same code path that returned [] from getProperties.

[bkonyi]

Thanks for the fix!