anchor.diff:    final VariableDeclaration catchErrorContinuationVariable =
         new VariableDeclaration(":async_op_error");
   
  +  LabeledStatement labeledBody;
  +
     ExpressionLifter expressionRewriter;
   
     AsyncRewriterBase(helper, enclosingFunction)
  
positive.code:     return null;
    }
  
    TreeNode visitReturnStatement(ReturnStatement stmt) {
      if (stmt.expression != null) {
        stmt.expression = expressionRewriter.rewrite(stmt.expression, statements)
          ..parent = stmt;
      }
      statements.add(stmt);
      return null;
    }
  
    TreeNode visitTryCatch(TryCatch stmt) {
      ++currentTryDepth;
      stmt.body = visitDelimited(stmt.body)..parent = stmt;
  
negative.code:     controllerVariable = new VariableDeclaration(":controller");
      statements.add(controllerVariable);
  
      setupAsyncContinuations(statements);
  
      // :controller = new _AsyncController(:async_op);
      var arguments =
  