Let Backend determine support for deferred loading.

pkg/compiler/lib/src/deferred_load.dart

 // Copyright (c) 2014, 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 deferred_load;
 
 import 'constants/expressions.dart';
 import 'constants/values.dart' show
     ConstantValue,
     ConstructedConstantValue,
@@ skipping 752 matching lines @@
                   ? previousDeferredImport
                   : import;
               compiler.reportError(failingImport.prefix,
                   MessageKind.DEFERRED_LIBRARY_DUPLICATE_PREFIX);
             }
             usedPrefixes.add(prefix);
           }
         }
       });
     }
-    Backend backend = compiler.backend;
-    if (isProgramSplit && backend is JavaScriptBackend) {
-      backend.registerCheckDeferredIsLoaded(compiler.globalDependencies);
-    }
-    if (isProgramSplit && backend is DartBackend) {
-      // TODO(sigurdm): Implement deferred loading for dart2dart.
-      compiler.reportWarning(
-          lastDeferred,
-          MessageKind.DEFERRED_LIBRARY_DART_2_DART);
-      isProgramSplit = false;
+    if (isProgramSplit) {
+      isProgramSplit = compiler.backend.registerDeferredLoading(
+            lastDeferred, compiler.globalDependencies);
     }
   }
 
   /// If [send] is a static send with a deferred element, returns the
   /// [PrefixElement] that the first prefix of the send resolves to.
   /// Otherwise returns null.
   ///
   /// Precondition: send must be static.
   ///
   /// Example:
@@ skipping 97 matching lines @@
         _importingLibrary = importingLibrary;
 
   String get importingLibraryName {
     String libraryName = _importingLibrary.getLibraryName();
     return libraryName == ""
       ? "<unnamed>"
       : libraryName;
   }
 
 }