Try to re-relativize this stuff

dart/build_rules/internal.bzl

@@ -126,6 +126,8 @@ def make_dart_context(label,
   if not lib_root:
     if label.workspace_root.startswith("external/"):
       lib_root = "%s/lib/" % label.workspace_root[len("external/"):]
+    elif not label.package:
+      lib_root = "lib/"
     else:
       lib_root = "%s/lib/" % label.package
 
@@ -206,13 +208,7 @@ def package_spec_action(ctx, dart_ctx, output):
   content = "# Generated by Bazel\n"
   for dc in dart_ctxs:
     relative_lib_root = _relative_path(dart_ctx.label.package, dc.lib_root)
-    if dc.label.workspace_root.startswith("external/"):
-      content += "%s:%s\n" % (dc.package, dc.lib_root)
-    else:
-      if dc.lib_root == '/lib/':
-        content += "%s:lib/\n" % (dc.package)
-      else:
-        content += "%s:%s\n" % (dc.package, relative_lib_root)
+    content += "%s:%s\n" % (dc.package, relative_lib_root)
 
   # Emit the package spec.
   ctx.file_action(
@@ -223,6 +219,8 @@ def package_spec_action(ctx, dart_ctx, output):
 
 def _relative_path(from_dir, to_path):
   """Returns the relative path from a directory to a path via the repo root."""
+  if not from_dir:
+    return to_path
   return "../" * (from_dir.count("/") + 1) + to_path