Enable causal stacktrace in kernel

pkg/kernel/lib/transformations/continuation.dart

Index: pkg/kernel/lib/transformations/continuation.dart
diff --git a/pkg/kernel/lib/transformations/continuation.dart b/pkg/kernel/lib/transformations/continuation.dart
index d8122e3bbb83af37ec97088e4c8e0449303172bf..10a96a9552f432a3073ac10df1a5e5f521302ce5 100644
--- a/pkg/kernel/lib/transformations/continuation.dart
+++ b/pkg/kernel/lib/transformations/continuation.dart
@@ -198,6 +198,8 @@ abstract class AsyncRewriterBase extends ContinuationRewriterBase {
       new VariableDeclaration(":async_op_then");
   final VariableDeclaration catchErrorContinuationVariable =
       new VariableDeclaration(":async_op_error");
+  final VariableDeclaration asyncStackTrace =
+      new VariableDeclaration(":async_stack_trace");
 
   LabeledStatement labeledBody;
 
@@ -215,6 +217,9 @@ abstract class AsyncRewriterBase extends ContinuationRewriterBase {
     // var :async_op_error;
     statements.add(catchErrorContinuationVariable);
 
+    // var :async_stack_trace;
+    statements.add(asyncStackTrace);
+
     // :async_op([:result, :exception, :stack_trace]) {
     //     modified <node.body>;
     // }