[generators] Add some explanation on forcing context allocation.

src/full-codegen/arm/full-codegen-arm.cc

Index: src/full-codegen/arm/full-codegen-arm.cc
diff --git a/src/full-codegen/arm/full-codegen-arm.cc b/src/full-codegen/arm/full-codegen-arm.cc
index ac647142f3b8ebc3fd8d8af32c5ca59ac3b92c7f..3c991d107cbac1fc7a7728dc3497a7d1e15ee044 100644
--- a/src/full-codegen/arm/full-codegen-arm.cc
+++ b/src/full-codegen/arm/full-codegen-arm.cc
@@ -1908,7 +1908,10 @@ void FullCodeGenerator::EmitGeneratorResume(Expression *generator,
   __ ldr(r2, FieldMemOperand(r1, JSGeneratorObject::kReceiverOffset));
   __ push(r2);
 
-  // Push holes for the rest of the arguments to the 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.
   __ ldr(r3, FieldMemOperand(r4, JSFunction::kSharedFunctionInfoOffset));
   __ ldr(r3,
          FieldMemOperand(r3, SharedFunctionInfo::kFormalParameterCountOffset));