Enhance DirectiveListener to collect import/exports with combinators.

pkg/front_end/tool/fasta_perf.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.
 
 /// An entrypoint used to run portions of fasta and measure its performance.
 library front_end.tool.fasta_perf;
 
 import 'dart:async';
 import 'dart:io';
 
@@ skipping 145 matching lines @@
 
   helper(start);
 }
 
 /// Parse [contents] as a Dart program and return the URIs that appear in its
 /// import, export, and part directives.
 Set<String> extractDirectiveUris(List<int> contents) {
   var listener = new DirectiveListenerWithNative();
   new TopLevelParser(listener).parseUnit(tokenize(contents));
   return new Set<String>()
-    ..addAll(listener.imports)
-    ..addAll(listener.exports)
+    ..addAll(listener.imports.map((d) => d.uri))
+    ..addAll(listener.exports.map((d) => d.uri))
     ..addAll(listener.parts);
 }
 
 class DirectiveListenerWithNative extends DirectiveListener {
   @override
   Token handleNativeClause(Token token) => skipNativeClause(token);
 }
 
 /// Parses every file in [files] and reports the time spent doing so.
 void parseFiles(Map<Uri, List<int>> files) {
@@ skipping 75 matching lines @@
 /// Report that metric [name] took [time] micro-seconds to process
 /// [inputSize] characters.
 void report(String name, int time) {
   var sb = new StringBuffer();
   var padding = ' ' * (20 - name.length);
   sb.write('$name:$padding $time us, ${time ~/ 1000} ms');
   var invSpeed = (time * 1000 / inputSize).toStringAsFixed(2);
   sb.write(', $invSpeed ns/char');
   print('$sb');
 }