Only report number of retained methods if we actually find an import of mirrors.

sdk/lib/_internal/compiler/implementation/js_backend/backend.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
index d157b569549862ab23235d589406930f0d2b206e..5376e1ee10b7bf178b969a24877c8e4a76304eb7 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
@@ -1193,13 +1193,7 @@ class JavaScriptBackend extends Backend {
     emitter.assembleProgram();
     int totalMethodCount = generatedCode.length;
     if (totalMethodCount != preMirrorsMethodCount) {
-      int mirrorCount = totalMethodCount - preMirrorsMethodCount;
-      double percentage = (mirrorCount / totalMethodCount) * 100;
-      compiler.reportHint(
-          compiler.mainApp, MessageKind.MIRROR_BLOAT,
-          {'count': mirrorCount,
-           'total': totalMethodCount,
-           'percentage': percentage.round()});
+      bool reportedHint = false;
       for (LibraryElement library in compiler.libraries.values) {
         if (library.isInternalLibrary) continue;
         for (LibraryTag tag in library.tags) {
@@ -1207,6 +1201,16 @@ class JavaScriptBackend extends Backend {
           if (importTag == null) continue;
           LibraryElement importedLibrary = library.getLibraryFromTag(tag);
           if (importedLibrary != compiler.mirrorsLibrary) continue;
+          if (!reportedHint) {
+            reportedHint = true;
+            int mirrorCount = totalMethodCount - preMirrorsMethodCount;
+            double percentage = (mirrorCount / totalMethodCount) * 100;
+            compiler.reportHint(
+                compiler.mainApp, MessageKind.MIRROR_BLOAT,
+                {'count': mirrorCount,
+                 'total': totalMethodCount,
+                 'percentage': percentage.round()});
+          }
           MessageKind kind =
               compiler.mirrorUsageAnalyzerTask.hasMirrorUsage(library)
               ? MessageKind.MIRROR_IMPORT