Don't crash for pre-1.12 global packages.

lib/src/global_packages.dart

Index: lib/src/global_packages.dart
diff --git a/lib/src/global_packages.dart b/lib/src/global_packages.dart
index ec0f5414def719698cf03de8e0b9e225a4f52308..c58ed54c63749028f79ee2b2a24d59a97e9ae064 100644
--- a/lib/src/global_packages.dart
+++ b/lib/src/global_packages.dart
@@ -353,7 +353,6 @@ class GlobalPackages {
     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       log.fine("$package:$executable is out of date and needs to be "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.");