[generators] Add some explanation on forcing context allocation.

src/full-codegen/x87/full-codegen-x87.cc

Index: src/full-codegen/x87/full-codegen-x87.cc
diff --git a/src/full-codegen/x87/full-codegen-x87.cc b/src/full-codegen/x87/full-codegen-x87.cc
index 446c343433c543a2e11cb5630245fa6bfcb65748..609516f9b24b08c5ecbd838917a4a55aa0ae110f 100644
--- a/src/full-codegen/x87/full-codegen-x87.cc
+++ b/src/full-codegen/x87/full-codegen-x87.cc
@@ -1815,7 +1815,10 @@ void FullCodeGenerator::EmitGeneratorResume(Expression *generator,
   // Push receiver.
   __ push(FieldOperand(ebx, 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.
   __ mov(edx, FieldOperand(edi, JSFunction::kSharedFunctionInfoOffset));
   __ mov(edx,
          FieldOperand(edx, SharedFunctionInfo::kFormalParameterCountOffset));