Implement correct label scope.

pkg/front_end/lib/src/fasta/builder/scope.dart

Index: pkg/front_end/lib/src/fasta/builder/scope.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/scope.dart b/pkg/front_end/lib/src/fasta/builder/scope.dart
index 73560239088d53f87ef8df5b6923d2df30baff8b..159d2ba66905d6c0076b4c7c0191503fe5ab7811 100644
--- a/pkg/front_end/lib/src/fasta/builder/scope.dart
+++ b/pkg/front_end/lib/src/fasta/builder/scope.dart
@@ -20,6 +20,8 @@ class Scope {
   /// succeed.
   final bool isModifiable;
 
+  Map<String, Builder> labels;
+
   Scope(this.local, this.parent, {this.isModifiable: true});
 
   Scope createNestedScope({bool isModifiable: true}) {
@@ -86,6 +88,21 @@ class Scope {
     return setter ? setterBuilder : getterBuilder;
   }
 
+  bool hasLocalLabel(String name) => labels != null && labels.containsKey(name);
+
+  void declareLabel(String name, Builder target) {
+    if (isModifiable) {
+      labels ??= <String, Builder>{};
+      labels[name] = target;
+    } else {
+      internalError("Can't extend an unmodifiable scope.");
+    }
+  }
+
+  Builder lookupLabel(String name) {
+    return (labels == null ? null : labels[name]) ?? parent?.lookupLabel(name);
+  }
+
   // TODO(ahe): Rename to extend or something.
   void operator []=(String name, Builder member) {
     if (isModifiable) {