dartdoc_checker throws assert re: html extension #64103
Description
When I run ./dev/bots/docs.sh, I get the following crash:
Unhandled exception:
'file:///Users/srawlins/code/flutter/dev/tools/dartdoc_checker.dart': Failed assertion: line 87 pos 10: 'path.extension(file.path) == 'html'': Scanned file is not HTML: "dev/docs/doc/api/dart-convert/Latin1Codec/encoder.html"; (.html)
#0 _AssertionError._doThrowNew (dart:core-patch/errors_patch.dart:46:39)
#1 _AssertionError._throwNew (dart:core-patch/errors_patch.dart:36:5)
#2 _scanFile (file:///Users/srawlins/code/flutter/dev/tools/dartdoc_checker.dart:87:10)
#3 checkForUnresolvedDirectives (file:///Users/srawlins/code/flutter/dev/tools/dartdoc_checker.dart:65:16)
#4 main (file:///Users/srawlins/code/flutter/dev/tools/dartdoc.dart:231:3)
<asynchronous suspension>
#5 _startIsolate.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:299:32)
#6 _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:168:12)
(I'd added a few comment lines, so the line numbers may be offset)
You can see that the assert checks against 'html', but above, on line 55, we're only checking files with extensions equal to '.html (note the dot).