Only parse the directives when rewriting imports.

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

Index: sdk/lib/_internal/pub/lib/src/barback/rewrite_import_transformer.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/rewrite_import_transformer.dart b/sdk/lib/_internal/pub/lib/src/barback/rewrite_import_transformer.dart
index 6f454b6519f14e2bcad1238447bfddb906139109..8392289e3e33d8d0d4d7b8815b84a6f36b7ace63 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/rewrite_import_transformer.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/rewrite_import_transformer.dart
@@ -17,7 +17,7 @@ class RewriteImportTransformer extends Transformer {
   Future apply(Transform transform) {
     return transform.primaryInput.readAsString().then((contents) {
       var collector = new _DirectiveCollector();
-      parseCompilationUnit(contents, name: transform.primaryInput.id.toString())
+      parseDirectives(contents, name: transform.primaryInput.id.toString())
           .accept(collector);
 
       var buffer = new StringBuffer();