Split MirrorsData methods and MirrorsDataImpl data by element kind.

tests/compiler/dart2js/sourcemaps/sourcemap_helper.dart

Index: tests/compiler/dart2js/sourcemaps/sourcemap_helper.dart
diff --git a/tests/compiler/dart2js/sourcemaps/sourcemap_helper.dart b/tests/compiler/dart2js/sourcemaps/sourcemap_helper.dart
index 85a19e012f2870d040838453e251dde945bd087c..72b0c13aff2552db6900b05b9e58b75ce442da9b 100644
--- a/tests/compiler/dart2js/sourcemaps/sourcemap_helper.dart
+++ b/tests/compiler/dart2js/sourcemaps/sourcemap_helper.dart
@@ -337,7 +337,8 @@ class SourceMapProcessor {
     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) {