Change code to remove two warnings that are reported when running build.py.

site/try/src/interaction_manager.dart

Index: site/try/src/interaction_manager.dart
diff --git a/site/try/src/interaction_manager.dart b/site/try/src/interaction_manager.dart
index 6106df05c24001e5e3abeae9024326b80a4c737e..2c566e33391e56da66852275cfc53c36caba195d 100644
--- a/site/try/src/interaction_manager.dart
+++ b/site/try/src/interaction_manager.dart
@@ -762,9 +762,10 @@ class InitialState extends InteractionState {
     if (context.isFirstCompile) {
       outputDiv.append(context.compilerConsole);
     }
+    var diagnostics = mainEditorPane.querySelectorAll('a.diagnostic');
     context.oldDiagnostics
         ..clear()
-        ..addAll(mainEditorPane.querySelectorAll('a.diagnostic'));
+        ..addAll(diagnostics);
   }
 
   void aboutToRun() {