Improve the heuristics for "pub get" is needed.

lib/src/executable.dart

 // Copyright (c) 2014, 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.
 
 library pub.executable;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:barback/barback.dart';
@@ skipping 64 matching lines @@
   // Ensure that there's a trailing extension.
   if (p.extension(executable) != ".dart") executable += ".dart";
 
   var localSnapshotPath = p.join(".pub", "bin", package,
       "$executable.snapshot");
   if (!isGlobal && fileExists(localSnapshotPath) &&
       // Dependencies are only snapshotted in release mode, since that's the
       // default mode for them to run. We can't run them in a different mode
       // using the snapshot.
       mode == BarbackMode.RELEASE) {
+    // Since we don't access the package graph, this doesn't happen
+    // automatically.
+    entrypoint.assertUpToDate();
+
     return _runCachedExecutable(entrypoint, localSnapshotPath, args,
         checked: checked);
   }
 
   // If the command has a path separator, then it's a path relative to the
   // root of the package. Otherwise, it's implicitly understood to be in
   // "bin".
   if (p.split(executable).length == 1) executable = p.join("bin", executable);
 
   var vmArgs = [];
@@ skipping 185 matching lines @@
 Future<int> _runCachedExecutable(Entrypoint entrypoint, String snapshotPath,
     List<String> args, {bool checked: false}) {
   return runSnapshot(snapshotPath, args,
       packagesFile: entrypoint.packagesFile,
       checked: checked,
       recompile: () {
     log.fine("Precompiled executable is out of date.");
     return entrypoint.precompileExecutables();
   });
 }