Pass an absolute package root to the VM.

lib/src/executable.dart

Index: lib/src/executable.dart
diff --git a/lib/src/executable.dart b/lib/src/executable.dart
index 863f306f672e01c03349298ddf37efffcd3faa6c..62c33a1c6c1499296c65354e5990e7c0bbe735a7 100644
--- a/lib/src/executable.dart
+++ b/lib/src/executable.dart
@@ -115,7 +115,12 @@ Future<int> runExecutable(Entrypoint entrypoint, String package,
   // example, it may not have the right packages directory itself.
   if (executableUrl.scheme == 'file' || executableUrl.scheme == '') {
     // TODO(nweiz): use a .packages file once sdk#23369 is fixed.
-    vmArgs.add('--package-root=${p.toUri(entrypoint.packagesDir)}');
+
+    // We use an absolute path here not because the VM insists but because it's
+    // helpful for the subprocess to be able to spawn Dart with
+    // Platform.executableArguments and have that work regardless of the working
+    // directory.
+    vmArgs.add('--package-root=${p.toUri(p.absolute(entrypoint.packagesDir))}');
   }
 
   vmArgs.add(executableUrl.toString());