Support removing dead packages with `pub install`.

utils/pub/utils.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.
 
 /**
  * Generic utility functions. Stuff that should possibly be in core.
  */
 #library('utils');
 
 #import('dart:crypto');
@@ skipping 63 matching lines @@
  */
 only(Iterable iter) {
   var iterator = iter.iterator();
   assert(iterator.hasNext());
   var obj = iterator.next();
   assert(!iterator.hasNext());
   return obj;
 }
 
 /**
+ * Returns a set containing all elements in [minuend] that are not in
+ * [subtrahend].
+ */
+Set setMinus(Collection minuend, Collection subtrahend) {
+  var minuendSet = new Set.from(minuend);
+  minuendSet.removeAll(subtrahend);
+  return minuendSet;
+}
+
+/**
  * Replace each instance of [matcher] in [source] with the return value of [fn].
  */
 String replace(String source, Pattern matcher, String fn(Match)) {
   var buffer = new StringBuffer();
   var start = 0;
   for (var match in matcher.allMatches(source)) {
     buffer.add(source.substring(start, match.start()));
     start = match.end();
     buffer.add(fn(match));
   }
@@ skipping 18 matching lines @@
   CryptoUtils.bytesToHex(new SHA1().update(source.charCodes()).digest());
 
 /**
  * Returns a [Future] that completes in [milliSeconds].
  */
 Future sleep(int milliSeconds) {
   var completer = new Completer();
   new Timer(milliSeconds, completer.complete);
   return completer.future;
 }