Examples command throws exception when pub serve encounters dart2js warning

[bencampbell-wf]

The dart_dev executable throws an exception when it encounters a dart2js warning when serving a non-dartium client. This exception causes the dart_dev process to exit leaving the child pub serve process running.

    [example] GET ....
    [Info from Dart2JS]:
    Compiling <omitted_path>/main.dart...
Unhandled exception:
Uncaught Error: [Dart2JS on <omitted_path>/main.dart]:
#0      _rootHandleUncaughtError.<anonymous closure> (dart:async/zone.dart:895)
#1      _microtaskLoop (dart:async/schedule_microtask.dart:43)
#2      _microtaskLoopEntry (dart:async/schedule_microtask.dart:52)
#3      _runPendingImmediateCallback (dart:isolate-patch/isolate_patch.dart:96)
#4      _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:149)

This seems to be an issue when a child process of dart_dev emits output to stderr.

Steps to Reproduce

• run pub run dart_dev examples with an example that includes code that throws a warning or hint in the dart2js compiler.
• serve a dartium client to confirm and confirm no error
• serve a non-dartium(chrome) client and confirm it throws an exception

[maxwellpeterson-wf]

This also happens when pub serve encounters a 404