Add KernelToElementMap.getSpannable to avoid most usage of KernelAstAdapter.getNode

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 81145270f5ab3a321c3b06d77b419c94f8709862..fc47021cd798faef63f692f913f16019d529f557 100644
--- a/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
+++ b/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
@@ -409,6 +409,11 @@ class KernelAstAdapter extends KernelToElementMapMixin {
     assert(constructorBody != null);
     return constructorBody;
   }
+
+  @override
+  Spannable getSpannable(MemberEntity member, ir.Node node) {
+    return getNode(node);
+  }
 }
 
 /// Visitor that converts kernel dart types into [ResolutionDartType].