Add a "Label" Node around an Identifier that is being used as a label.

lib/compiler/implementation/tree/unparser.dart

Index: lib/compiler/implementation/tree/unparser.dart
diff --git a/lib/compiler/implementation/tree/unparser.dart b/lib/compiler/implementation/tree/unparser.dart
index 9aaad5f83ccd6deef031c158a2a2ba0282cca91f..17852756cf2781af25b2609512899147492ac7cb 100644
--- a/lib/compiler/implementation/tree/unparser.dart
+++ b/lib/compiler/implementation/tree/unparser.dart
@@ -307,9 +307,13 @@ class Unparser implements Visitor {
     visit(node.body);
   }
 
+  visitLabel(Label node) {
+    visit(node.identifier);
+    add(node.colonToken.value);
+   }
+
   visitLabeledStatement(LabeledStatement node) {
     visit(node.label);
-    add(node.colonToken.value);
     sb.add(' ');
     visit(node.statement);
   }