Extract BackendUsage, MirrorsData, and CheckedModeHelpers from Backend.

pkg/compiler/lib/src/mirrors_used.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.mirrors_used;
 
 import 'common/tasks.dart' show CompilerTask;
 import 'common.dart';
 import 'compile_time_constants.dart' show ConstantCompiler;
 import 'compiler.dart' show Compiler;
@@ skipping 81 matching lines @@
   /// Collect @MirrorsUsed annotations in all libraries.  Called by the
   /// compiler after all libraries are loaded, but before resolution.
   void analyzeUsage(LibraryElement mainApp) {
     if (mainApp == null || compiler.commonElements.mirrorsLibrary == null) {
       return;
     }
     measure(analyzer.run);
     List<String> symbols = analyzer.mergedMirrorUsage.symbols;
     List<Element> targets = analyzer.mergedMirrorUsage.targets;
     List<Element> metaTargets = analyzer.mergedMirrorUsage.metaTargets;
-    compiler.backend.registerMirrorUsage(
+    compiler.backend.mirrorsData.registerMirrorUsage(
         symbols == null ? null : new Set<String>.from(symbols),
         targets == null ? null : new Set<Element>.from(targets),
         metaTargets == null ? null : new Set<Element>.from(metaTargets));
     librariesWithUsage = analyzer.librariesWithUsage;
   }
 
   /// Is there a @MirrorsUsed annotation in the library of [element]?  Used by
   /// the resolver to suppress hints about using new Symbol or
   /// MirrorSystem.getName.
   bool hasMirrorUsage(Element element) {
@@ skipping 453 matching lines @@
       // @MirrorsUsed(targets: fisk)
       //                       ^^^^
       //
       // Instead of saying 'fisk' should pretty print the problematic constant
       // value.
       return spannable;
     }
     return node;
   }
 }