Throw when reflecting on elements not covered by a `MirrorsUsed` annotation.

sdk/lib/_internal/compiler/implementation/js_backend/namer.dart

 // Copyright (c) 2011, 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.
 
 part of js_backend;
 
 /**
  * Assigns JavaScript identifiers to Dart variables, class-names and members.
  */
 class Namer implements ClosureNamer {
@@ skipping 164 matching lines @@
     return _jsVariableReserved;
   }
 
   final String CURRENT_ISOLATE;
   String get GLOBAL_OBJECT => CURRENT_ISOLATE;
 
   final String getterPrefix = r'get$';
   final String setterPrefix = r'set$';
   final String metadataField = '@';
   final String callCatchAllName = r'call$catchAll';
+  final String reflectableField = r'$reflectable';
 
   /**
    * Map from top-level or static elements to their unique identifiers provided
    * by [getName].
    *
    * Invariant: Keys must be declaration elements.
    */
   final Compiler compiler;
   final Map<Element, String> globals;
   final Map<String, LibraryElement> shortPrivateNameOwners;
@@ skipping 41 matching lines @@
    */
   String get STATIC_CLOSURE_NAME_NAME => r'$name';
   SourceString get closureInvocationSelectorName => Compiler.CALL_OPERATOR_NAME;
   bool get shouldMinify => false;
 
   String getNameForJsGetName(Node node, String name) {
     switch (name) {
       case 'GETTER_PREFIX': return getterPrefix;
       case 'SETTER_PREFIX': return setterPrefix;
       case 'CALL_CATCH_ALL': return callCatchAllName;
+      case 'REFLECTABLE': return reflectableField;
       default:
         compiler.reportError(
             node, MessageKind.GENERIC,
             {'text': 'Error: Namer has no name for "$name".'});
         return 'BROKEN';
     }
   }
 
   bool isReserved(String name) => name == isolateName;
 
@@ skipping 1021 matching lines @@
         if (!first) {
           sb.write('_');
         }
         sb.write('_');
         visit(link.head);
         first = true;
       }
     }
   }
 }