Split before "deferred" if present.

lib/src/source_visitor.dart

Index: lib/src/source_visitor.dart
diff --git a/lib/src/source_visitor.dart b/lib/src/source_visitor.dart
index 402842e81b016b2d8b08d836b0c5ca5aa80eae61..4a86da4bf9c02ff563aa0297ee5dcde28c79df5c 100644
--- a/lib/src/source_visitor.dart
+++ b/lib/src/source_visitor.dart
@@ -968,9 +968,14 @@ class SourceVisitor implements AstVisitor {
       token(node.keyword);
       space();
       visit(node.uri);
-      token(node.deferredKeyword, before: space);
-      token(node.asKeyword, before: soloSplit, after: space);
-      visit(node.prefix);
+
+      if (node.asKeyword != null) {
+        soloSplit();
+        token(node.deferredKeyword, after: space);
+        token(node.asKeyword);
+        space();
+        visit(node.prefix);
+      }
 
       builder.startRule(new CombinatorRule());
       visitNodes(node.combinators);