Move hasRuntimeTypeSupport, hasIsolateSupport, and hasFunctionApplySupport to JavaScriptBackend.

pkg/compiler/lib/src/common/backend_api.dart

Index: pkg/compiler/lib/src/common/backend_api.dart
diff --git a/pkg/compiler/lib/src/common/backend_api.dart b/pkg/compiler/lib/src/common/backend_api.dart
index c24a2a6f227607a077e08898229b4bfd1ef97048..e978d7e9cf58bd428ede7e6f280aee2c566e445e 100644
--- a/pkg/compiler/lib/src/common/backend_api.dart
+++ b/pkg/compiler/lib/src/common/backend_api.dart
@@ -314,8 +314,9 @@ abstract class Backend extends Target {
   /// times, but [onQueueClosed] is only called once.
   void onQueueClosed() {}
 
-  /// Called when the compiler starts running the codegen enqueuer.
-  void onCodegenStart() {}
+  /// Called when the compiler starts running the codegen enqueuer. The
+  /// [WorldImpact] of enabled backend features is returned.
+  WorldImpact onCodegenStart() => const WorldImpact();
 
   // Does this element belong in the output
   bool shouldOutput(Element element) => true;