[dart2dart] In renamer only add import prefix for top-level elements.

lib/compiler/implementation/dart_backend/placeholder_collector.dart

Index: lib/compiler/implementation/dart_backend/placeholder_collector.dart
diff --git a/lib/compiler/implementation/dart_backend/placeholder_collector.dart b/lib/compiler/implementation/dart_backend/placeholder_collector.dart
index 31fa905f98e0ea3b5837c86ff938740de6b3f339..52840994c4e4b986fc28e681d24d1a4139a99cb3 100644
--- a/lib/compiler/implementation/dart_backend/placeholder_collector.dart
+++ b/lib/compiler/implementation/dart_backend/placeholder_collector.dart
@@ -205,6 +205,10 @@ class PlaceholderCollector extends AbstractVisitor {
     assert(element !== null);
     if (element === entryFunction) return;
     if (element.getLibrary() === coreLibrary) return;
+    if (isDartCoreLib(compiler, element.getLibrary())
+        && !element.isTopLevel()) {
+      return;
+    }
     if (element == compiler.types.dynamicType.element) {
       internalError(
           'Should never make element placeholder for dynamic type element',