Fix strong mode warning

pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart

Index: pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
diff --git a/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart b/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
index 6585f30b8e0bd93dfb4dcac1a660436787c78f52..bb78b62ff45ec5f67ddd92ba8f1f3675aa0e47d2 100644
--- a/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
+++ b/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
@@ -416,7 +416,7 @@ class DartTypeConverter extends ir.DartTypeVisitor<ResolutionDartType> {
   }
 }
 
-class KernelJumpTarget extends JumpTarget {
+class KernelJumpTarget extends JumpTarget<ast.Node> {
   static int index = 0;
 
   /// Pointer to the actual executable statements that a jump target refers to.