[vm/bytecode] Capture :controller_stream variable in async* functions

Test: runtime/observatory/tests/service/async_star_step_out_test.dart

Change-Id: If6dec1b5005b0736edaf85b974581f294e602f40
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/114463
Reviewed-by: Régis Crelier <[email protected]>

Author: alexmarkov

pkg/kernel/lib/transformations/continuation.dart

@@ -16,6 +16,7 @@ class ContinuationVariables {
   static const awaitJumpVar = ':await_jump_var';
   static const awaitContextVar = ':await_ctx_var';
   static const asyncStackTraceVar = ':async_stack_trace';
+  static const controllerStreamVar = ':controller_stream';
   static const exceptionParam = ':exception';
   static const stackTraceParam = ':stack_trace';
 
@@ -919,7 +920,7 @@ class AsyncStarFunctionRewriter extends AsyncRewriterBase {
 
     // dynamic :controller_stream;
     VariableDeclaration controllerStreamVariable =
-        new VariableDeclaration(":controller_stream");
+        new VariableDeclaration(ContinuationVariables.controllerStreamVar);
     statements.add(controllerStreamVariable);
 
     setupAsyncContinuations(statements);

pkg/vm/lib/bytecode/local_vars.dart

@@ -390,6 +390,13 @@ class _ScopeBuilder extends RecursiveVisitor<Null> {
         _useVariable(_currentFrame.parent
             .getSyntheticVar(ContinuationVariables.awaitContextVar));
 
+        // Debugger looks for :controller_stream variable among captured
+        // variables in a context, so make sure to capture it.
+        if (_currentFrame.parent.dartAsyncMarker == AsyncMarker.AsyncStar) {
+          _useVariable(_currentFrame.parent
+              .getSyntheticVar(ContinuationVariables.controllerStreamVar));
+        }
+
         if (locals.options.causalAsyncStacks &&
             (_currentFrame.parent.dartAsyncMarker == AsyncMarker.Async ||
                 _currentFrame.parent.dartAsyncMarker ==