make dart2js_info strong-mode clean

bin/inference/print_summary.dart

Index: bin/inference/print_summary.dart
diff --git a/bin/inference/print_summary.dart b/bin/inference/print_summary.dart
index a0dbc0b1eeb2d1c82cd9a3e7652f69b81a663c12..45a7612e06b27e0f1b2863b6bee2288ead656751 100644
--- a/bin/inference/print_summary.dart
+++ b/bin/inference/print_summary.dart
@@ -81,10 +81,11 @@ class _Counter extends RecursiveInfoVisitor {
     totals.addFrom(currentBundleTotals);
   }
 
-  visitFunction(FunctionInfo function) {
+  Null visitFunction(FunctionInfo function) {
     var measurements = function.measurements;
-    if (measurements == null) return;
+    if (measurements == null) return null;
     currentBundleTotals.addFrom(measurements);
+    return null;
   }
 }