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

lib/compiler/implementation/ssa/builder.dart

Index: lib/compiler/implementation/ssa/builder.dart
diff --git a/lib/compiler/implementation/ssa/builder.dart b/lib/compiler/implementation/ssa/builder.dart
index e8d852fa6b589eb91fbc1d910f5e07bf80079673..e867f6c16aa00e90ce73d9ab1ddfe1c83ec018f1 100644
--- a/lib/compiler/implementation/ssa/builder.dart
+++ b/lib/compiler/implementation/ssa/builder.dart
@@ -2815,6 +2815,10 @@ class SsaBuilder implements Visitor {
     handleLoop(node, buildInitializer, buildCondition, () {}, buildBody);
   }
 
+  visitLabel(Label node) {
+    compiler.internalError('SsaBuilder.visitLabel', node: node);
+  }
+
   visitLabeledStatement(LabeledStatement node) {
     Statement body = node.getBody();
     if (body is Loop || body is SwitchStatement) {