Reapply "Gracefully handle if loaded dill program has no mainMethod set."

pkg/compiler/lib/src/library_loader.dart

 // Copyright (c) 2012, 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 dart2js.library_loader;
 
 import 'dart:async';
 
 import 'common/names.dart' show Uris;
 import 'common/tasks.dart' show CompilerTask, Measurer;
@@ skipping 855 matching lines @@
       ir.Program program = new ir.Program();
       // Hack because the existing file has a terminating 0 and the
       // BinaryBuilder doesn't expect that.
       var bytes = new List<int>.from(script.file.slowUtf8ZeroTerminatedBytes());
       bytes.removeLast();
       new BinaryBuilder(bytes).readProgram(program);
       return measure(() {
         _worldBuilder = new KernelWorldBuilder(reporter, program);
         program.libraries.forEach((ir.Library library) => _allLoadedLibraries
             .add(_worldBuilder.lookupLibrary(library.importUri)));
-        // TODO(efortuna): Handle `prgram.mainMethod == null` gracefully.
+        LibraryEntity rootLibrary = null;
+        if (program.mainMethod != null) {
+          rootLibrary = _worldBuilder
+              .lookupLibrary(program.mainMethod.enclosingLibrary.importUri);
+        }
         return new _LoadedLibrariesAdapter(
-            _worldBuilder
-                .lookupLibrary(program.mainMethod.enclosingLibrary.importUri),
-            _allLoadedLibraries,
-            _worldBuilder);
+            rootLibrary, _allLoadedLibraries, _worldBuilder);
       });
     });
   }
 
   KernelWorldBuilder get worldBuilder => _worldBuilder;
 
   void reset({bool reuseLibrary(LibraryElement library)}) {
     throw new UnimplementedError('_DillLibraryLoaderTask.reset');
   }
 
@@ skipping 785 matching lines @@
 }
 
 /// API used by the library loader to synchronously scan a library or
 /// compilation unit and ensure that their library tags are computed.
 abstract class ElementScanner {
   void scanLibrary(LibraryElement library);
   void scanUnit(CompilationUnitElement unit);
 }
 
 const _reuseLibrarySubtaskName = "Reuse library";