Allow main to be null for modular tranformation.

pkg/kernel/lib/transformations/setup_builtin_library.dart

Index: pkg/kernel/lib/transformations/setup_builtin_library.dart
diff --git a/pkg/kernel/lib/transformations/setup_builtin_library.dart b/pkg/kernel/lib/transformations/setup_builtin_library.dart
index cc4149a74e908c2e7f6f27f23959ee9d10cede9b..c04d05091eec28a044e00bb26479657905bb54aa 100644
--- a/pkg/kernel/lib/transformations/setup_builtin_library.dart
+++ b/pkg/kernel/lib/transformations/setup_builtin_library.dart
@@ -13,6 +13,8 @@ Program transformProgram(Program program,
     {String libraryUri: 'dart:_builtin'}) {
   Procedure mainMethod = program.mainMethod;
 
+  if (mainMethod == null) return program;
+
   Library builtinLibrary;
   for (Library library in program.libraries) {
     if (library.importUri.toString() == libraryUri) {