[interpreter] Fix debug stepping for generators.

src/parsing/parser.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/parsing/parser.h"
 
 #include "src/api.h"
 #include "src/ast/ast.h"
 #include "src/ast/ast-expression-rewriter.h"
 #include "src/ast/ast-expression-visitor.h"
@@ skipping 4723 matching lines @@
           factory()->NewBlock(nullptr, 3, false, RelocInfo::kNoPosition);
 
       {
         Expression* allocation = BuildCreateJSGeneratorObject(pos, kind);
         VariableProxy* init_proxy = factory()->NewVariableProxy(
             function_state_->generator_object_variable());
         Assignment* assignment = factory()->NewAssignment(
             Token::INIT, init_proxy, allocation, RelocInfo::kNoPosition);
         VariableProxy* get_proxy = factory()->NewVariableProxy(
             function_state_->generator_object_variable());
-        Yield* yield =
-            factory()->NewYield(get_proxy, assignment, RelocInfo::kNoPosition);
+        // The position of the yield is important for reporting the exception
+        // caused by calling the .throw method on a generator suspended at the
+        // initial yield (i.e. right after generator instantiation).
+        Yield* yield = factory()->NewYield(get_proxy, assignment,
+                                           scope_->start_position());
         try_block->statements()->Add(
             factory()->NewExpressionStatement(yield, RelocInfo::kNoPosition),
             zone());
       }
 
       ParseStatementList(try_block->statements(), Token::RBRACE, CHECK_OK);
 
       Statement* final_return = factory()->NewReturnStatement(
           BuildIteratorResult(nullptr, true), RelocInfo::kNoPosition);
       try_block->statements()->Add(final_return, zone());
@@ skipping 2209 matching lines @@
                         try_block, target);
     final_loop = target;
   }
 
   return final_loop;
 }
 
 
 }  // namespace internal
 }  // namespace v8