Gracefully handle if loaded dill program has no mainMethod set.

pkg/compiler/lib/src/library_loader.dart

Index: pkg/compiler/lib/src/library_loader.dart
diff --git a/pkg/compiler/lib/src/library_loader.dart b/pkg/compiler/lib/src/library_loader.dart
index 0903f6e84aaf494005a61d4150989ddd4230f410..9caf929497f187d9dfbe50b2a1b65ee05318526f 100644
--- a/pkg/compiler/lib/src/library_loader.dart
+++ b/pkg/compiler/lib/src/library_loader.dart
@@ -873,12 +873,13 @@ class _DillLibraryLoaderTask extends CompilerTask implements LibraryLoaderTask {
         _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);
       });
     });
   }