Implement correct label scope.

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

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.scope;
 
 import 'builder.dart' show Builder, MixedAccessor;
 
 import '../errors.dart' show internalError;
 
 class Scope {
   /// Names declared in this scope.
   final Map<String, Builder> local;
 
   /// The scope that this scope is nested within, or `null` if this is the top
   /// level scope.
   final Scope parent;
 
   /// Indicates whether an attempt to declare new names in this scope should
   /// succeed.
   final bool isModifiable;
 
+  Map<String, Builder> labels;
+
   Scope(this.local, this.parent, {this.isModifiable: true});
 
   Scope createNestedScope({bool isModifiable: true}) {
     return new Scope(<String, Builder>{}, this, isModifiable: isModifiable);
   }
 
   Builder lookup(String name, int charOffset, Uri fileUri) {
     Builder builder = local[name];
     if (builder != null) {
       if (builder.next != null) {
@@ skipping 46 matching lines @@
       } else {
         return new AmbiguousBuilder(builder, charOffset, fileUri);
       }
       current = current.next;
     }
     assert(getterBuilder != null);
     assert(setterBuilder != null);
     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) {
       local[name] = member;
     } else {
       internalError("Can't extend an unmodifiable scope.");
     }
   }
 }
 
@@ skipping 33 matching lines @@
 class AmbiguousBuilder extends Builder {
   final Builder builder;
 
   AmbiguousBuilder(this.builder, int charOffset, Uri fileUri)
       : super(null, charOffset, fileUri);
 
   get target => null;
 
   bool get hasProblem => true;
 }