Attach source range information to parsed YAML nodes.

sdk/lib/_internal/pub/lib/src/barback.dart

Index: sdk/lib/_internal/pub/lib/src/barback.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback.dart b/sdk/lib/_internal/pub/lib/src/barback.dart
index af5e0c45927b8597c40d748283ef9c076dd8d816..83b63de92b8f8cbc582889014261b7e1be6908fe 100644
--- a/sdk/lib/_internal/pub/lib/src/barback.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback.dart
@@ -140,6 +140,9 @@ class TransformerId {
     if (configuration == null) {
       configuration = {};
     } else {
+      // Don't write to the immutable YAML map.
+      configuration = new Map.from(configuration);
+
       // Pull out the exclusions/inclusions.
       includes = parseField("\$include");
       excludes = parseField("\$exclude");