[ Observatory ] Fix bad import and bin/observatory.dart crash

Change-Id: I299facbd77522ccdb90dfdd2871f7bf23103b975
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/434840
Reviewed-by: Derek Xu <derekx@google.com>
Commit-Queue: Ben Konyi <bkonyi@google.com>
Auto-Submit: Ben Konyi <bkonyi@google.com>
Reviewed-by: Jessy Yameogo <yjessy@google.com>

pkg/observatory/bin/observatory.dart

@@ -90,11 +90,9 @@ ${result.stderr}
     ]);
     final uriCompleter = Completer<String>();
     final uriRegExp = RegExp('Serving `web` on (http://.*)');
-    late StreamSubscription<String> sub;
-    sub = process.stdout.transform(utf8.decoder).listen((e) {
+    final sub = process.stdout.transform(utf8.decoder).listen((e) {
       if (uriRegExp.hasMatch(e)) {
         uriCompleter.complete(uriRegExp.firstMatch(e)!.group(1));
-        sub.cancel();
       }
     });
 
@@ -105,6 +103,11 @@ ${result.stderr}
       Chrome.start(<String>[observatoryUri]);
     }
     await process.exitCode;
+
+    // Don't cancel this stream until the process has exited as it will close
+    // the file descriptor for stdout and cause a crash when webdev tries to
+    // write logs.
+    await sub.cancel();
   }
 
   String _findObservatoryProjectRoot() {

pkg/observatory/web/main.dart

@@ -4,7 +4,7 @@
 
 import 'package:web/web.dart';
 import 'package:logging/logging.dart';
-import '../lib/elements.dart';
+import 'package:observatory/elements.dart';
 import 'package:stack_trace/stack_trace.dart';
 
 main() async {