Let Backend determine support for deferred loading.

pkg/compiler/lib/src/dart_backend/backend.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.
 
 part of dart_backend;
 
 // TODO(ahe): This class is simply wrong.  This backend should use
 // elements when it can, not AST nodes.  Perhaps a [Map<Element,
 // TreeElements>] is what is needed.
 class ElementAst {
@@ skipping 294 matching lines @@
               } else {
                 registry.registerDynamicInvocation(selector);
               }
             });
           }
         }
       }
     }
 
   }
+
+  @override
+  bool registerDeferredLoading(Spannable node, Registry registry) {
+    // TODO(sigurdm): Implement deferred loading for dart2dart.
+    compiler.reportWarning(node, MessageKind.DEFERRED_LIBRARY_DART_2_DART);
+    return false;
+  }
 }
 
 class DartResolutionCallbacks extends ResolutionCallbacks {
   final DartBackend backend;
 
   DartResolutionCallbacks(this.backend);
 
   void onTypeLiteral(DartType type, Registry registry) {
     if (type.isInterfaceType) {
       backend.usedTypeLiterals.add(type.element);
@@ skipping 185 matching lines @@
 class _ElementAstCreationContext implements ElementAstCreationContext {
   final Compiler compiler;
   final ConstantSystem constantSystem;
 
   _ElementAstCreationContext(this.compiler, this.constantSystem);
 
   DartTypes get dartTypes => compiler.types;
 
   InternalErrorFunction get internalError => compiler.internalError;
 }