Add debug names to scopes.

pkg/front_end/lib/src/fasta/source/source_library_builder.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.source_library_builder;
 
 import 'package:kernel/ast.dart' show ProcedureKind;
 
 import '../../base/resolve_relative_uri.dart' show resolveRelativeUri;
 
@@ skipping 564 matching lines @@
 
   final Map<String, Builder> setters;
 
   final List<T> types = <T>[];
 
   String name;
 
   final Map<ProcedureBuilder, DeclarationBuilder<T>> factoryDeclarations;
 
   DeclarationBuilder(this.members, this.setters, this.constructors,
-      this.factoryDeclarations, this.name, this.parent);
+      this.factoryDeclarations, this.name, this.parent) {
+    assert(name != null);
+  }
 
   DeclarationBuilder.library()
-      : this(<String, Builder>{}, <String, Builder>{}, null, null, null, null);
+      : this(<String, Builder>{}, <String, Builder>{}, null, null, "library",
+            null);
 
   DeclarationBuilder createNested(String name, bool hasMembers) {
     return new DeclarationBuilder<T>(
         hasMembers ? <String, MemberBuilder>{} : null,
         hasMembers ? <String, MemberBuilder>{} : null,
         hasMembers ? <String, MemberBuilder>{} : null,
         <ProcedureBuilder, DeclarationBuilder<T>>{},
         name,
         this);
   }
@@ skipping 55 matching lines @@
 
   /// Called to register [procedure] as a factory whose types are collected in
   /// [factoryDeclaration]. Later, once the class has been built, we can
   /// synthesize type variables on the factory matching the class'.
   void addFactoryDeclaration(
       ProcedureBuilder procedure, DeclarationBuilder<T> factoryDeclaration) {
     factoryDeclarations[procedure] = factoryDeclaration;
   }
 
   Scope toScope(Scope parent) {
-    return new Scope(members, setters, parent, isModifiable: false);
+    return new Scope(members, setters, parent, name, isModifiable: false);
   }
 }