Implement nested switches and missing switch continue targets.

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;
 
+  Map<String, Builder> forwardDeclaredLabels;
+
   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 57 matching lines @@
 
   void declareLabel(String name, Builder target) {
     if (isModifiable) {
       labels ??= <String, Builder>{};
       labels[name] = target;
     } else {
       internalError("Can't extend an unmodifiable scope.");
     }
   }
 
+  void forwardDeclareLabel(String name, Builder target) {
+    declareLabel(name, target);
+    forwardDeclaredLabels ??= <String, Builder>{};
+    forwardDeclaredLabels[name] = target;
+  }
+
+  void claimLabel(String name) {
+    forwardDeclaredLabels.remove(name);
+    if (forwardDeclaredLabels.length == 0) {
+      forwardDeclaredLabels = null;
+    }
+  }
+
+  Map<String, Builder> get unclaimedForwardDeclarations {
+    return forwardDeclaredLabels;
+  }
+
   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 37 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;
 }