Avoid completing a non-nullable Completer with a nullable value (#3328)

Implicitly passing a nullable value to a Completer of a non-nullable type can lead to surprising null-asserting exceptions. See dart-lang/sdk#53253 for more details. In this PR, we add an explicit null-assertion, which makes this call in-line with the general concepts of null safety.

Fixes flutter/flutter#137294

dashboard/lib/service/dev_cocoon.dart

@@ -35,7 +35,7 @@ class _PausedCommitStatus {
 
   void complete() {
     assert(_pausedStatus != null);
-    _completer.complete(_pausedStatus);
+    _completer.complete(_pausedStatus!);
     _pausedStatus = null;
   }
 }