Dart analysis server crashes (JSON readtable error 69)

Hi! When running

`dart-server-show-hover` on some code, I get the following in `*Messages*`:

```
next-line: End of buffer
Dart analysis server crashed, restarting.
json-read: JSON readtable error: 69
```

And setting debug on, I see in `*Backtrace*`:

```
Debugger entered--Lisp error: (json-readtable-error 69)
signal(json-readtable-error (69))
json-read()
json-read-from-string("Error: Error when reading '../../dart-sdk/sdk/sdk/bin/snapshots/analysis_server.dart.snapshot': No such file or directory")
dart-server--analysis-server-process-filter(#s(dart-server--analysis-server :process # :buffer #) "Error: $
#f(compiled-function (_ string) #)(# "Error: Error when reading '../../dart-sdk/sdk/sdk/bin/snapshots/analysis_server.dar$
read-event(nil t 2)
sit-for(2)
execute-extended-command(nil "dart-server-show-hover" nil)
funcall-interactively(execute-extended-command nil "dart-server-show-hover" nil)
call-interactively(execute-extended-command nil nil)
command-execute(execute-extended-command)
```

Version:

```
dart-server 20190817.1254 installed
```

Not sure what the issue is. Any idea?

Start with dart-server-show-hover and dart-server--analysis-server-process-filter, then reproduce the missing analysis_server.dart.snapshot error using the reported dart-server 20190817.1254 setup. Check how the process output is parsed when it contains the shown non-JSON error; done means the command no longer crashes with a json-readtable-error and reports the underlying server failure appropriately.

Written by the indexing model from the issue text.