Extract BackendUsage, MirrorsData, and CheckedModeHelpers from Backend.

pkg/compiler/lib/src/js_emitter/full_emitter/container_builder.dart

 // Copyright (c) 2013, 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 dart2js.js_emitter.full_emitter.container_builder;
 
 import '../../constants/values.dart';
 import '../../elements/elements.dart'
     show
         Element,
@@ skipping 133 matching lines @@
           ? js("null")
           : js.quoteName(tearOffName))
       ..add(js.number(requiredParameterCount))
       ..add(js.number(optionalParameterCount))
       ..add(memberTypeExpression == null ? js("null") : memberTypeExpression)
       ..addAll(task.metadataCollector.reifyDefaultArguments(member));
 
     if (canBeReflected || canBeApplied) {
       parameters.forEachParameter((Element parameter) {
         expressions.add(task.metadataCollector.reifyName(parameter.name));
-        if (backend.mustRetainMetadata) {
+        if (backend.mirrorsData.mustRetainMetadata) {
           Iterable<jsAst.Expression> metadataIndices =
               parameter.metadata.map((MetadataAnnotation annotation) {
             ConstantValue constant =
                 backend.constants.getConstantValueForMetadata(annotation);
             backend.constants.addCompileTimeConstantForEmission(constant);
             return task.metadataCollector.reifyMetadata(annotation);
           });
           expressions.add(new jsAst.ArrayInitializer(metadataIndices.toList()));
         }
       });
@@ skipping 19 matching lines @@
     jsAst.ArrayInitializer arrayInit =
         new jsAst.ArrayInitializer(expressions.toList());
     compiler.dumpInfoTask
         .registerElementAst(member, builder.addProperty(name, arrayInit));
   }
 
   void addMemberField(Field field, ClassBuilder builder) {
     // For now, do nothing.
   }
 }