[generators] Add some explanation on forcing context allocation.

src/full-codegen/x64/full-codegen-x64.cc

Index: src/full-codegen/x64/full-codegen-x64.cc
diff --git a/src/full-codegen/x64/full-codegen-x64.cc b/src/full-codegen/x64/full-codegen-x64.cc
index 285e3d7ff38b283c6367eac1de7c254d4e5279d9..88dce608859e11f616b3ac1bd7a5604eae5fea3d 100644
--- a/src/full-codegen/x64/full-codegen-x64.cc
+++ b/src/full-codegen/x64/full-codegen-x64.cc
@@ -1848,7 +1848,10 @@ void FullCodeGenerator::EmitGeneratorResume(
   // Push receiver.
   __ Push(FieldOperand(rbx, JSGeneratorObject::kReceiverOffset));
 
-  // Push holes for arguments to generator function.
+  // Push holes for arguments to generator function. Since the parser forced
+  // context allocation for any variables in generators, the actual argument
+  // values have already been copied into the context and these dummy values
+  // will never be used.
   __ movp(rdx, FieldOperand(rdi, JSFunction::kSharedFunctionInfoOffset));
   __ LoadSharedFunctionInfoSpecialField(rdx, rdx,
       SharedFunctionInfo::kFormalParameterCountOffset);