Add FrontendStrategy.commonElements and remove Backend.commonElements

pkg/compiler/lib/src/kernel/task.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.
 
 import '../common/names.dart';
 import '../common/tasks.dart' show CompilerTask;
 import '../compiler.dart';
 import '../elements/elements.dart';
 import 'kernel.dart';
 import 'package:kernel/ast.dart' as ir;
@@ skipping 21 matching lines @@
         program = buildProgram(_compiler.mainApp);
       });
 
   /// Builds the kernel IR program for the main function exported from
   /// [library].
   ///
   /// May enqueue more elements to the resolution queue.
   ir.Program buildProgram(LibraryElement library) {
     var main = library.findExported(Identifiers.main);
     if (main == null) {
-      main = _compiler.backend.helperForMissingMain();
+      main = _compiler.frontendStrategy.commonElements.missingMain;
     }
     return new ir.Program(
         libraries: kernel.libraryDependencies(library.canonicalUri))
       ..mainMethod = kernel.functionToIr(main);
   }
 }