Avoid using FunctionSignature in builder_kernel

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 c04d89bd24958853b8cc1d49e99afa0fea8625f3..09c9ede9b7ed0ce2cd116f109c9355c4433c4b6d 100644
--- a/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
+++ b/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
@@ -219,10 +219,6 @@ class KernelAstAdapter extends KernelToElementMapMixin
     return getElement(variable) as LocalElement;
   }
 
-  FunctionSignature getFunctionSignature(ir.FunctionNode function) {
-    return getElement(function).asFunctionElement().functionSignature;
-  }
-
   // Is the member a lazy initialized static or top-level member?
   bool isLazyStatic(ir.Member member) {
     if (member is ir.Field) {