Improve verbose trace output (#9104)

Related #8876

packages/flutter_tools/lib/src/devfs.dart

@@ -339,14 +339,16 @@ class DevFS {
   Uri get baseUri => _baseUri;
 
   Future<Uri> create() async {
+    printTrace('DevFS: Creating new filesystem on the device ($_baseUri)');
     _baseUri = await _operations.create(fsName);
     printTrace('DevFS: Created new filesystem on the device ($_baseUri)');
     return _baseUri;
   }
 
-  Future<dynamic> destroy() {
+  Future<Null> destroy() async {
+    printTrace('DevFS: Deleting filesystem on the device ($_baseUri)');
+    await _operations.destroy(fsName);
     printTrace('DevFS: Deleted filesystem on the device ($_baseUri)');
-    return _operations.destroy(fsName);
   }
 
   /// Update files on the device and return the number of bytes sync'd

packages/flutter_tools/lib/src/resident_runner.dart

@@ -226,7 +226,19 @@ abstract class ResidentRunner {
       throwToolExit('No Flutter view is available');
 
     // Listen for service protocol connection to close.
-    vmService.done.whenComplete(appFinished);
+    vmService.done.then<Null>(
+        _serviceProtocolDone,
+        onError: _serviceProtocolError).whenComplete(appFinished);
+  }
+
+  Future<Null> _serviceProtocolDone(dynamic object) {
+    printTrace('Service protocol connection closed.');
+    return new Future<Null>.value(object);
+  }
+
+  Future<Null> _serviceProtocolError(dynamic error, StackTrace stack) {
+    printTrace('Service protocol connection closed with an error: $error\n$stack');
+    return new Future<Null>.error(error, stack);
   }
 
   /// Returns [true] if the input has been handled by this function.