Log only valid occurences of 'All native types used ...'.

pkg/compiler/lib/src/native/enqueue.dart

Index: pkg/compiler/lib/src/native/enqueue.dart
diff --git a/pkg/compiler/lib/src/native/enqueue.dart b/pkg/compiler/lib/src/native/enqueue.dart
index 300a65078008d9a6442f57b958ce9bf3c494d4d8..bd37b4c88f000c4628dacc30300614f111ee1641 100644
--- a/pkg/compiler/lib/src/native/enqueue.dart
+++ b/pkg/compiler/lib/src/native/enqueue.dart
@@ -487,7 +487,7 @@ abstract class NativeEnqueuerBase implements NativeEnqueuer {
 
     // Give an info so that library developers can compile with -v to find why
     // all the native classes are included.
-    if (unusedBefore == matchingClasses.length) {
+    if (unusedBefore > 0 && unusedBefore == matchingClasses.length) {
       reporter.log('All native types marked as used due to $cause.');
     }
   }