Extract BackendUsage, MirrorsData, and CheckedModeHelpers from Backend.

pkg/compiler/lib/src/js_emitter/program_builder/field_visitor.dart

 // Copyright (c) 2015, 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 dart2js.js_emitter.program_builder;
 
 /**
  * [member] is a field (instance, static, or top level).
  *
  * [name] is the field name that the [Namer] has picked for this field's
@@ skipping 122 matching lines @@
       // allowed on fields that are in [element] we don't need to visit
       // superclasses for non-instantiated classes.
       cls.implementation.forEachInstanceField(visitField,
           includeSuperAndInjectedMembers: isInstantiated);
     }
   }
 
   bool fieldNeedsGetter(VariableElement field) {
     assert(field.isField);
     if (fieldAccessNeverThrows(field)) return false;
-    if (backend.shouldRetainGetter(field)) return true;
+    if (backend.mirrorsData.shouldRetainGetter(field)) return true;
     return field.isClassMember &&
         compiler.codegenWorldBuilder.hasInvokedGetter(field, closedWorld);
   }
 
   bool fieldNeedsSetter(VariableElement field) {
     assert(field.isField);
     if (fieldAccessNeverThrows(field)) return false;
     if (field.isFinal || field.isConst) return false;
-    if (backend.shouldRetainSetter(field)) return true;
+    if (backend.mirrorsData.shouldRetainSetter(field)) return true;
     return field.isClassMember &&
         compiler.codegenWorldBuilder.hasInvokedSetter(field, closedWorld);
   }
 
   static bool fieldAccessNeverThrows(VariableElement field) {
     return
         // We never access a field in a closure (a captured variable) without
         // knowing that it is there.  Therefore we don't need to use a getter
         // (that will throw if the getter method is missing), but can always
         // access the field directly.
         field is ClosureFieldElement;
   }
 
   bool canAvoidGeneratedCheckedSetter(VariableElement member) {
     // We never generate accessors for top-level/static fields.
     if (!member.isInstanceMember) return true;
     ResolutionDartType type = member.type;
     return type.treatAsDynamic || type.isObject;
   }
 }