Split MirrorsData methods and MirrorsDataImpl data by element kind.

tests/compiler/dart2js/sourcemaps/sourcemap_helper.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.
 
 library sourcemap.helper;
 
 import 'dart:async';
 import 'dart:io';
 import 'package:compiler/compiler_new.dart';
 import 'package:compiler/src/apiimpl.dart' as api;
@@ skipping 319 matching lines @@
     RecordingSourceInformationStrategy strategy =
         new RecordingSourceInformationStrategy(
             backend.sourceInformationStrategy);
     backend.sourceInformationStrategy = strategy;
     await compiler.run(inputUri);
 
     SourceMapInfo mainSourceMapInfo;
     Map<Element, SourceMapInfo> elementSourceMapInfos =
         <Element, SourceMapInfo>{};
     if (perElement) {
-      backend.generatedCode.forEach((Element element, js.Expression node) {
+      backend.generatedCode
+          .forEach((MemberElement element, js.Expression node) {
         RecordedSourceInformationProcess subProcess =
             strategy.subProcessForNode(node);
         if (subProcess == null) {
           // TODO(johnniwinther): Find out when this is happening and if it
           // is benign. (Known to happen for `bool#fromString`)
           print('No subProcess found for $element');
           return;
         }
         LocationMap nodeMap = subProcess.nodeToSourceLocationsMap;
         String code = subProcess.code;
@@ skipping 218 matching lines @@
       absoluteUri = base.resolveUri(uri);
     } else {
       absoluteUri = base.resolve(uri);
     }
     return sourceFiles.putIfAbsent(absoluteUri, () {
       String text = new File.fromUri(absoluteUri).readAsStringSync();
       return new StringSourceFile.fromUri(absoluteUri, text);
     });
   }
 }