Allow main to be null for modular tranformation.

pkg/kernel/lib/transformations/setup_builtin_library.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library kernel.transformations.setup_builtin_library;
 
 import '../ast.dart';
 
 // The DartVM has a special `dart:_builtin` library which exposes a
 // `_getMainClosure()` method.  We need to change this method to return a
 // closure of `main()`.
 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) {
       builtinLibrary = library;
       break;
     }
   }
 
   if (builtinLibrary == null) {
     throw new Exception('Could not find "dart:_builtin" library');
@@ skipping 10 matching lines @@
   if (getMainClosure == null) {
     throw new Exception('Could not find "_getMainClosure" in "$libraryUri"');
   }
 
   var returnMainStatement = new ReturnStatement(new StaticGet(mainMethod));
   getMainClosure.body = returnMainStatement;
   returnMainStatement.parent = getMainClosure;
 
   return program;
 }