Do not create placeholders for elements that won't be later renamed.

lib/compiler/implementation/dart_backend/renamer.dart

Index: lib/compiler/implementation/dart_backend/renamer.dart
diff --git a/lib/compiler/implementation/dart_backend/renamer.dart b/lib/compiler/implementation/dart_backend/renamer.dart
index 6cb47f76933ce788de7d7a7ba4b5556e60b75b84..6193ea683a0e8ee97b82f4948ddcf0c1a463f5b7 100644
--- a/lib/compiler/implementation/dart_backend/renamer.dart
+++ b/lib/compiler/implementation/dart_backend/renamer.dart
@@ -38,8 +38,6 @@ void renamePlaceholders(
     // local identifiers.
     String originalName = element.name.slowToString();
     LibraryElement library = element.getLibrary();
-    if (library === compiler.coreLibrary
-        || element == compiler.mainApp.find(Compiler.MAIN)) return originalName;
     if (isDartCoreLib(compiler, library)) {
       final prefix =
           imports.putIfAbsent(library, () => generateUniqueName('p'));