Don't crash for pre-1.12 global packages.

lib/src/global_packages.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.global_packages;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
@@ skipping 335 matching lines @@
 
     // Unless the user overrides the verbosity, we want to filter out the
     // normal pub output shown while loading the environment.
     if (log.verbosity == log.Verbosity.NORMAL) {
       log.verbosity = log.Verbosity.WARNING;
     }
 
     var snapshotPath = p.join(binDir, '$executable.dart.snapshot');
     return exe.runSnapshot(snapshotPath, args,
         checked: checked,
-        packagesFile: _getPackagesFilePath(package),
         recompile: () async {
       log.fine("$package:$executable is out of date and needs to be "
           "recompiled.");
       await _precompileExecutables(
           find(package).packageGraph.entrypoint, package);
     });
   }
 
   /// Gets the path to the lock file for an activated cached package with
   /// [name].
@@ skipping 417 matching lines @@
   }
 
   /// Returns the value of the property named [name] in the bin stub script
   /// [source].
   String _binStubProperty(String source, String name) {
     var pattern = new RegExp(quoteRegExp(name) + r": ([a-zA-Z0-9_-]+)");
     var match = pattern.firstMatch(source);
     return match == null ? null : match[1];
   }
 }