Add "load from .dill" file capability and run a white-box test.

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 26468e128a9ed80bc1f8dc1e164eb97996f92871..765188f72a6e54cf2255757884de4b7773b65c36 100644
--- a/pkg/compiler/lib/src/js_backend/mirrors_analysis.dart
+++ b/pkg/compiler/lib/src/js_backend/mirrors_analysis.dart
@@ -120,7 +120,8 @@ class MirrorsResolutionAnalysisImpl implements MirrorsResolutionAnalysis {
     if (_mirrorsData.mustRetainMetadata) {
       _reporter.log('Retaining metadata.');
 
-      _compiler.libraryLoader.libraries.forEach(_mirrorsData.retainMetadataOf);
+      (_compiler.libraryLoader.libraries as Iterable<LibraryElement>)
+          .forEach(_mirrorsData.retainMetadataOf);
 
       if (!enqueuer.queueIsClosed) {
         /// Register the constant value of [metadata] as live in resolution.
@@ -243,7 +244,8 @@ class MirrorsCodegenAnalysisImpl implements MirrorsCodegenAnalysis {
     if (_mirrorsData.mustRetainMetadata) {
       _reporter.log('Retaining metadata.');
 
-      _compiler.libraryLoader.libraries.forEach(_mirrorsData.retainMetadataOf);
+      (_compiler.libraryLoader.libraries as Iterable<LibraryElement>)
+          .forEach(_mirrorsData.retainMetadataOf);
 
       for (Dependency dependency in _metadataConstants) {
         _impactBuilder