Improve the heuristics for "pub get" is needed.

lib/src/io.dart

 // Copyright (c) 2013, 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.
 
 /// Helper functionality to make working with IO easier.
 library pub.io;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:convert';
@@ skipping 813 matching lines @@
     executable = "cmd";
   }
 
   log.process(executable, args, workingDir == null ? '.' : workingDir);
 
   return fn(executable, args,
       workingDirectory: workingDir,
       environment: environment);
 }
 
+/// Updates [path]'s modification time.
+void touch(String path) {
+  var file = new File(path).openSync(mode: FileMode.APPEND);
+  var originalLength = file.lengthSync();
+  file.writeByteSync(0);
+  file.truncateSync(originalLength);
+  file.closeSync();
+}
+
 /// Creates a temporary directory and passes its path to [fn].
 ///
 /// Once the [Future] returned by [fn] completes, the temporary directory and
 /// all its contents are deleted. [fn] can also return `null`, in which case
 /// the temporary directory is deleted immediately afterwards.
 ///
 /// Returns a future that completes to the value that the future returned from
 /// [fn] completes to.
 Future withTempDir(Future fn(String path)) {
   return new Future.sync(() {
@@ skipping 207 matching lines @@
 
   // TODO(rnystrom): Remove this and change to returning one string.
   static List<String> _toLines(String output) {
     var lines = splitLines(output);
     if (!lines.isEmpty && lines.last == "") lines.removeLast();
     return lines;
   }
 
   bool get success => exitCode == exit_codes.SUCCESS;
 }