Analyzer gets stuck analyzing this invalid code

[DanTup]

If I create a file with the following content:

void foo(String s) {
  switch (s) {
    case 'foo') {
    print('foo');
  } else {
    print('bar');
  }
}

The server becomes unresponsive, or running dart analyze never completes.

[bwilkerson]

The problem appears to be at https://github.com/dart-lang/sdk/blob/main/pkg/_fe_analyzer_shared/lib/src/scanner/abstract_scanner.dart#L442. The loop never terminates because candidate == candidate.previous. I'm guessing that the bug is wherever the synthetic token is being created / inserted into the stream. Whatever the case, it's the parser that's stuck in an infinite loop.