DAS: improve error-throwing code to include JSON

Most calls to `jsonDecoder.mismatch` include the 3rd argument. I add
more here, so that the error can say, "I expected this field in some
JSON, but instead the JSON looks like this: ..."

Change-Id: I7d1944624e1fe758d9732d71db6571a579dc4690
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/447360
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Samuel Rawlins <[email protected]>

Author: srawlins