Enhance DirectiveListener to collect import/exports with combinators.

pkg/front_end/lib/src/dependency_grapher_impl.dart

 // Copyright (c) 2017, 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 'dart:async';
 
 import 'package:front_end/dependency_grapher.dart';
 import 'package:front_end/src/async_dependency_walker.dart';
 import 'package:front_end/src/base/processed_options.dart';
 import 'package:front_end/src/fasta/parser.dart';
@@ skipping 107 matching lines @@
       }
     }
 
     for (var part in listener.parts) {
       // TODO(paulberry): when we support SDK libraries, we'll need more
       // complex logic here to find SDK parts correctly.
       library.parts.add(uri.resolve(part));
     }
 
     for (var dep in listener.imports) {
-      handleDependency(uri.resolve(dep));
+      handleDependency(uri.resolve(dep.uri));
     }
 
     for (var dep in listener.exports) {
-      handleDependency(uri.resolve(dep));
+      handleDependency(uri.resolve(dep.uri));
     }
 
     if (!coreUriFound) {
       handleDependency(dartCoreUri);
     }
     return dependencies;
   }
 }