Add debug names to scopes.

pkg/front_end/lib/src/fasta/source/outline_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.outline_builder;
 
 import 'package:kernel/ast.dart' show ProcedureKind;
 
 import '../../scanner/token.dart' show Token;
 
@@ skipping 249 matching lines @@
     debugEvent("endLibraryName");
     popCharOffset();
     String name = pop();
     List<MetadataBuilder> metadata = pop();
     library.name = name;
     library.metadata = metadata;
   }
 
   @override
   void beginClassOrNamedMixinApplication(Token token) {
-    library.beginNestedDeclaration(null);
+    library.beginNestedDeclaration("class or mixin application");
   }
 
   @override
   void beginClassDeclaration(Token begin, Token name) {
     library.currentDeclaration.name = name.lexeme;
   }
 
   @override
   void beginNamedMixinApplication(Token beginToken, Token name) {
     library.currentDeclaration.name = name.lexeme;
@@ skipping 671 matching lines @@
       docToken = docToken.next;
     }
     return buffer.toString();
   }
 
   @override
   void debugEvent(String name) {
     // printEvent(name);
   }
 }