Do not assign positions to parser-generated desugarings.

src/hydrogen.h

 // 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.
 
 #ifndef V8_HYDROGEN_H_
 #define V8_HYDROGEN_H_
 
 #include "src/v8.h"
 
 #include "src/accessors.h"
@@ skipping 1850 matching lines @@
   HInstruction* BuildCheckPrototypeMaps(Handle<JSObject> prototype,
                                         Handle<JSObject> holder);
 
   HInstruction* BuildGetNativeContext(HValue* closure);
   HInstruction* BuildGetNativeContext();
   HInstruction* BuildGetScriptContext(int context_index);
   HInstruction* BuildGetArrayFunction();
 
  protected:
   void SetSourcePosition(int position) {
-    DCHECK(position != RelocInfo::kNoPosition);

[adamk, 2015/03/24 15:57:41]

Was this for/let desugaring really the first place to run into this DCHECK?

[Dmitry Lomov (no reviews), 2015/03/24 16:42:52]

Seems to be. This for/let desugaring added comparisons and hydrogen randomly
calls SetSourcePosition for these nodes. It does not call SetSourcePostition for
other nodes.
The entire notion of source positons is needed for --hydrogen-track-positions
flag, which is mostly unused.

-    position_.set_position(position - start_position_);
+    if (position != RelocInfo::kNoPosition) {
+      position_.set_position(position - start_position_);
+    }
+    // Otherwise position remains unknown.
   }
 
   void EnterInlinedSource(int start_position, int id) {
     if (top_info()->is_tracking_positions()) {
       start_position_ = start_position;
       position_.set_inlining_id(id);
     }
   }
 
   // Convert the given absolute offset from the start of the script to
@@ skipping 1147 matching lines @@
   }
 
  private:
   HGraphBuilder* builder_;
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_H_