Don't crash for pre-1.12 global packages.

lib/src/executable.dart

Index: lib/src/executable.dart
diff --git a/lib/src/executable.dart b/lib/src/executable.dart
index 749277ff148e5121d06ded95b81147381aaa78cd..65b276ee4cdbf5b5c430aa4d527d54fd687ca7a9 100644
--- a/lib/src/executable.dart
+++ b/lib/src/executable.dart
@@ -111,7 +111,12 @@ Future<int> runExecutable(Entrypoint entrypoint, String package,
   // 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