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

lib/compiler/implementation/typechecker.dart

Index: lib/compiler/implementation/typechecker.dart
diff --git a/lib/compiler/implementation/typechecker.dart b/lib/compiler/implementation/typechecker.dart
index 8c69f55ddf23a1076295f490ad9c5933d78a8b98..a5cdb5fa2a98be5d40d941e48e52bad90d9a5ec3 100644
--- a/lib/compiler/implementation/typechecker.dart
+++ b/lib/compiler/implementation/typechecker.dart
@@ -703,6 +703,8 @@ class TypeCheckerVisitor implements Visitor<Type> {
     return bodyType.join(StatementType.NOT_RETURNING);
   }
 
+  visitLabel(Label node) { }
+
   visitLabeledStatement(LabeledStatement node) {
     return node.statement.accept(this);
   }