The full log is below, but it looks like what's happening is that the file is being deleted while "pub serve" is reading it. I'm assigning this to Bob because there's a related TODO in the file to update the test once pub serve supports file watching, which it does now.

FAIL: responds with a 404 for missing files
  Caught The schedule had 1 errors:
  ScheduleError:
  | FileSystemException: Cannot delete file, path = C:\Users\CHROME2\AppData\Local\Temp\pub_4c0fb32e-5c8d-11e3-b2d3-005056030208\myapp\lib\nope.dart (OS Error: The process cannot access the file because it is being used by another process.
  
  | , errno = 32)
  
  Stack chain:
  | file_impl.dart 545 _File.throwIfError
  | file_impl.dart 307 _File._deleteSync
  | file_system_entity.dart 339 FileSystemEntity.deleteSync
  | sdk\lib_internal\pub\lib\src\io.dart 313:30 deleteEntry
  | sdk\lib_internal\pub\test\serve\missing_file_test.dart 45:18 main.<fn>.<fn>
  | dart:isolate _RawReceivePortImpl._handleMessage
  
  Error detected during task in queue "tasks":
  * creating directory:
  | myapp
  | |-- async descriptor
  | |-- asset
  | | '-- nope.png
  | |-- lib
  | | '-- nope.dart
  | '-- web
  | '-- index.html
  * starting process 'pub serve'
  * reading the next stdout line from process 'pub serve'
  > delete files
  |
  | Stack chain:
  | | package:scheduled_test/scheduled_test.dart 291:33 schedule
  | | sdk\lib_internal\pub\test\serve\missing_file_test.dart 43:13 main.<fn>
  | | sdk\lib_internal\pub\test\test_pub.dart 348:9 _integration.<fn>
  | | dart:isolate _RawReceivePortImpl._handleMessage
  * request index.html
  * request packages/myapp/nope.dart
  * request assets/myapp/nope.png
  * request dir/packages/myapp/nope.dart
  * request dir/assets/myapp/nope.png
  * waiting for process 'pub serve' to die
  ================================================================================
  
  Debug info:
  Results of running 'pub serve':
  Process was killed prematurely.
  Standard output:
  | MSG : Serving myapp on http://127.0.0.1:62217
  Standard error:
  | FINE: Pub 0.1.2+3
  | IO : Spawning cmd /c ver
  | IO : Finished ver. Exit code 0.stdout:Microsoft Windows [Version 6.1.7601]Nothing output on stderr.
  | IO : Listing directory C:\Users\CHROME2\AppData\Local\Temp\pub_4c0fb32e-5c8d-11e3-b2d3-005056030208\myapp\asset.

0 PASSED, 1 FAILED, 0 ERRORS