Delete dead code (this was used for source-mirrors, which are now gone).

pkg/compiler/lib/src/compiler.dart

 // Copyright (c) 2012, 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.compiler_base;
 
 import 'dart:async' show EventSink, Future;
 
 import '../compiler_new.dart' as api;
 import 'cache_strategy.dart' show CacheStrategy;
@@ skipping 181 matching lines @@
 
   // Initialized after mirrorSystemClass has been resolved.
   FunctionElement mirrorSystemGetNameFunction;
 
   // Initialized when mirrorsUsedClass has been resolved.
   FunctionElement mirrorsUsedConstructor;
 
   // Initialized when dart:mirrors is loaded.
   ClassElement deferredLibraryClass;
 
-  /// Document class from dart:mirrors.
-  ClassElement documentClass;
   Element identicalFunction;
   Element loadLibraryFunction;
   Element functionApplyMethod;
 
   // TODO(zarah): Remove this map and incorporate compile-time errors
   // in the model.
   /// Tracks elements with compile-time errors.
   final Map<Element, List<DiagnosticMessage>> elementsWithCompileTimeErrors =
       new Map<Element, List<DiagnosticMessage>>();
 
@@ skipping 343 matching lines @@
         reporter.reportWarningMessage(
             NO_LOCATION_SPANNABLE, MessageKind.IMPORT_EXPERIMENTAL_MIRRORS, {
           'importChain': importChains
               .join(MessageTemplate.IMPORT_EXPERIMENTAL_MIRRORS_PADDING)
         });
       }
 
       coreClasses.functionClass.ensureResolved(resolution);
       functionApplyMethod =
           coreClasses.functionClass.lookupLocalMember('apply');
-
-      if (options.preserveComments) {
-        return libraryLoader
-            .loadLibrary(Uris.dart_mirrors)
-            .then((LibraryElement libraryElement) {
-          documentClass = libraryElement.find('Comment');
-        });
-      }
     }).then((_) => backend.onLibrariesLoaded(loadedLibraries));
   }
 
   Element findRequiredElement(LibraryElement library, String name) {
     var element = library.find(name);
     if (element == null) {
       reporter.internalError(
           library,
           "The library '${library.canonicalUri}' does not contain required "
           "element: '$name'.");
@@ skipping 1601 matching lines @@
   _ElementScanner(this.scanner);
   void scanLibrary(LibraryElement library) => scanner.scanLibrary(library);
   void scanUnit(CompilationUnitElement unit) => scanner.scan(unit);
 }
 
 class _EmptyEnvironment implements Environment {
   const _EmptyEnvironment();
 
   String valueOf(String key) => null;
 }