Don't crash for pre-1.12 global packages.

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 93 matching lines @@
     if (package != entrypoint.root.name) {
       message += " in package ${log.bold(package)}";
     }
     log.error("$message.");
     return exit_codes.NO_INPUT;
   }
 
   // If we're running an executable directly from the filesystem, make sure that
   // it knows where to load the packages. If it's a dependency's executable, for
   // example, it may not have the right packages directory itself.
-  if (executableUrl.scheme == 'file' || executableUrl.scheme == '') {
+  //
+  // We don't do this for global executables because older ones may not have a
+  // `.packages` file generated. If they do, the VM's logic will find it
+  // automatically.
+  if (!isGlobal &&
+      (executableUrl.scheme == 'file' || executableUrl.scheme == '')) {
     // We use an absolute path here not because the VM insists but because it's
     // helpful for the subprocess to be able to spawn Dart with
     // Platform.executableArguments and have that work regardless of the working
     // directory.
     vmArgs.add('--packages=${p.toUri(p.absolute(entrypoint.packagesFile))}');
   }
 
   vmArgs.add(executableUrl.toString());
   vmArgs.addAll(args);
 
@@ skipping 150 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     log.fine("Precompiled executable is out of date.");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();
   });
 }