Reuse RuntimeTypesNeedBuilderImpl and _RuntimeTypesNeed for kernel based elements

pkg/compiler/lib/src/kernel/kernel_strategy.dart

Index: pkg/compiler/lib/src/kernel/kernel_strategy.dart
diff --git a/pkg/compiler/lib/src/kernel/kernel_strategy.dart b/pkg/compiler/lib/src/kernel/kernel_strategy.dart
index 6269627a97b87ec5f7c26cda96c5b609bb7a8e76..3f7474bd778fc0f0d74fe53cdb64318ce69a7bdd 100644
--- a/pkg/compiler/lib/src/kernel/kernel_strategy.dart
+++ b/pkg/compiler/lib/src/kernel/kernel_strategy.dart
@@ -99,7 +99,8 @@ class KernelFrontEndStrategy implements FrontEndStrategy {
   }
 
   RuntimeTypesNeedBuilder createRuntimeTypesNeedBuilder() {
-    return new RuntimeTypesNeedBuilderImpl();
+    return new RuntimeTypesNeedBuilderImpl(
+        elementEnvironment, elementMap.types);
   }
 
   ResolutionWorldBuilder createResolutionWorldBuilder(
@@ -141,27 +142,6 @@ class KernelWorkItem implements ResolutionWorkItem {
   }
 }
 
-/// Mock implementation of [RuntimeTypesNeedBuilder].
-class RuntimeTypesNeedBuilderImpl implements RuntimeTypesNeedBuilder {
-  @override
-  void registerClassUsingTypeVariableExpression(ClassEntity cls) {}
-
-  @override
-  RuntimeTypesNeed computeRuntimeTypesNeed(
-      ResolutionWorldBuilder resolutionWorldBuilder,
-      ClosedWorld closedWorld,
-      DartTypes types,
-      CommonElements commonElements,
-      BackendUsage backendUsage,
-      {bool enableTypeAssertions}) {
-    throw new UnimplementedError(
-        'RuntimeTypesNeedBuilderImpl.computeRuntimeTypesNeed');
-  }
-
-  @override
-  void registerRtiDependency(ClassEntity element, ClassEntity dependency) {}
-}
-
 /// Mock implementation of [MirrorsDataImpl].
 class MirrorsDataBuilderImpl extends MirrorsDataImpl {
   MirrorsDataBuilderImpl() : super(null, null, null);