Split MirrorsData methods and MirrorsDataImpl data by element kind.

pkg/compiler/lib/src/js_backend/mirrors_analysis.dart

Index: pkg/compiler/lib/src/js_backend/mirrors_analysis.dart
diff --git a/pkg/compiler/lib/src/js_backend/mirrors_analysis.dart b/pkg/compiler/lib/src/js_backend/mirrors_analysis.dart
index 26468e128a9ed80bc1f8dc1e164eb97996f92871..5f62b6e5998c5e58f58e827e099e296f02bfc9b5 100644
--- a/pkg/compiler/lib/src/js_backend/mirrors_analysis.dart
+++ b/pkg/compiler/lib/src/js_backend/mirrors_analysis.dart
@@ -73,14 +73,17 @@ class MirrorsResolutionAnalysisImpl implements MirrorsResolutionAnalysis {
       });
     }
 
-    for (Element target in _mirrorsData.targetsUsed) {
-      if (target == null) continue;
+    for (MemberElement target in _mirrorsData.membersInMirrorsUsedTargets) {
       if (target.isField) {
         staticFields.add(target);
-      } else if (target.isLibrary || target.isClass) {
-        addFieldsInContainer(target);
       }
     }
+    for (ClassElement target in _mirrorsData.classesInMirrorsUsedTargets) {
+      addFieldsInContainer(target);
+    }
+    for (LibraryElement target in _mirrorsData.librariesInMirrorsUsedTargets) {
+      addFieldsInContainer(target);
+    }
     return staticFields;
   }
 
@@ -107,7 +110,9 @@ class MirrorsResolutionAnalysisImpl implements MirrorsResolutionAnalysis {
     if (_mirrorsData.isTreeShakingDisabled) {
       enqueuer.applyImpact(_computeImpactForReflectiveElements(recentClasses,
           enqueuer.processedClasses, _compiler.libraryLoader.libraries));
-    } else if (!_mirrorsData.targetsUsed.isEmpty) {
+    } else if (_mirrorsData.membersInMirrorsUsedTargets.isNotEmpty ||
+        _mirrorsData.classesInMirrorsUsedTargets.isNotEmpty ||
+        _mirrorsData.librariesInMirrorsUsedTargets.isNotEmpty) {
       // Add all static elements (not classes) that have been requested for
       // reflection. If there is no mirror-usage these are probably not
       // necessary, but the backend relies on them being resolved.
@@ -120,7 +125,8 @@ class MirrorsResolutionAnalysisImpl implements MirrorsResolutionAnalysis {
     if (_mirrorsData.mustRetainMetadata) {
       _reporter.log('Retaining metadata.');
 
-      _compiler.libraryLoader.libraries.forEach(_mirrorsData.retainMetadataOf);
+      _compiler.libraryLoader.libraries
+          .forEach(_mirrorsData.retainMetadataOfLibrary);
 
       if (!enqueuer.queueIsClosed) {
         /// Register the constant value of [metadata] as live in resolution.
@@ -243,7 +249,8 @@ class MirrorsCodegenAnalysisImpl implements MirrorsCodegenAnalysis {
     if (_mirrorsData.mustRetainMetadata) {
       _reporter.log('Retaining metadata.');
 
-      _compiler.libraryLoader.libraries.forEach(_mirrorsData.retainMetadataOf);
+      _compiler.libraryLoader.libraries
+          .forEach(_mirrorsData.retainMetadataOfLibrary);
 
       for (Dependency dependency in _metadataConstants) {
         _impactBuilder
@@ -383,7 +390,7 @@ class MirrorsHandler {
   void _enqueueReflectiveSpecialClasses() {
     Iterable<ClassElement> classes = _backend.classesRequiredForReflection;
     for (ClassElement cls in classes) {
-      if (_backend.mirrorsData.referencedFromMirrorSystem(cls)) {
+      if (_backend.mirrorsData.isClassReferencedFromMirrorSystem(cls)) {
         _logEnqueueReflectiveAction(cls);
         cls.ensureResolved(_resolution);
         impactBuilder