Implement nested switches and missing switch continue targets.

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 159d2ba66905d6c0076b4c7c0191503fe5ab7811..10e5c39c864c2817993f03e92f5700b05b310062 100644
--- a/pkg/front_end/lib/src/fasta/builder/scope.dart
+++ b/pkg/front_end/lib/src/fasta/builder/scope.dart
@@ -22,6 +22,8 @@ class Scope {
 
   Map<String, Builder> labels;
 
+  Map<String, Builder> forwardDeclaredLabels;
+
   Scope(this.local, this.parent, {this.isModifiable: true});
 
   Scope createNestedScope({bool isModifiable: true}) {
@@ -99,6 +101,23 @@ class 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);
   }