Avoid adding constants for emission during resolution.

pkg/compiler/lib/src/js_backend/mirrors_analysis.dart

Index: pkg/compiler/lib/src/js_backend/mirrors_analysis.dart
diff --git a/pkg/compiler/lib/src/js_backend/mirrors_analysis.dart b/pkg/compiler/lib/src/js_backend/mirrors_analysis.dart
index b3617d8c57d5e4069ec09f5a08ae590c4b0bc210..6bc41cc82f3e19144a0dade020f311773be6bcc1 100644
--- a/pkg/compiler/lib/src/js_backend/mirrors_analysis.dart
+++ b/pkg/compiler/lib/src/js_backend/mirrors_analysis.dart
@@ -126,8 +126,10 @@ class MirrorsResolutionAnalysisImpl implements MirrorsResolutionAnalysis {
     if (_mirrorsData.mustRetainMetadata) {
       _reporter.log('Retaining metadata.');
 
-      (_compiler.libraryLoader.libraries as Iterable<LibraryElement>)
-          .forEach(_mirrorsData.retainMetadataOfLibrary);
+      for (LibraryEntity library in _compiler.libraryLoader.libraries) {
+        _mirrorsData.retainMetadataOfLibrary(library,
+            addForEmission: !enqueuer.isResolutionQueue);
+      }
 
       if (!enqueuer.queueIsClosed) {
         /// Register the constant value of [metadata] as live in resolution.