Move Compiler.libraries to LibraryLoader.

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 d161ec809ffc411ff67fda9be6d7cb3dd3a742e2..0ca7a05c9a0e0f0f138f46d7d7f4f0a1c6d3b871 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
@@ -1193,7 +1193,7 @@ class JavaScriptBackend extends Backend {
           {'count': mirrorCount,
            'total': totalMethodCount,
            'percentage': percentage.round()});
-      for (LibraryElement library in compiler.libraries.values) {
+      for (LibraryElement library in compiler.libraryLoader.libraries) {
         if (library.isInternalLibrary) continue;
         for (LibraryTag tag in library.tags) {
           Import importTag = tag.asImport();
@@ -1924,7 +1924,7 @@ class JavaScriptBackend extends Backend {
     if (mustRetainMetadata) {
       compiler.log('Retaining metadata.');
 
-      compiler.libraries.values.forEach(retainMetadataOf);
+      compiler.libraryLoader.libraries.forEach(retainMetadataOf);
       for (Dependency dependency in metadataConstants) {
         registerCompileTimeConstant(
             dependency.constant, dependency.registry);