Improve mismatch exception, wrapping field names in quotes

When hitting a protocol error, I saw some errors that were confusing
for a moment because of the lack of quotes, for example:

> Invalid parameter 'params'. Expected to be files; found \"...

This CL doesn't actually improve the situation _much_, but it at least
puts quotes around `files`, to indicate it is a property or something
similar.

Change-Id: Ic12d063b427661025cd4418951c58abf3394df3f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/449220
Commit-Queue: Samuel Rawlins <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: srawlins