Extend assembler with ability to produce comments for the generated code.

runtime/vm/code_generator_x64.h

Index: runtime/vm/code_generator_x64.h
diff --git a/runtime/vm/code_generator_x64.h b/runtime/vm/code_generator_x64.h
index c89fce76585b030abf1e2aac8da443b7e1022389..c60b8033447af3657b00762a5574bcd91e6d0dd6 100644
--- a/runtime/vm/code_generator_x64.h
+++ b/runtime/vm/code_generator_x64.h
@@ -41,6 +41,8 @@ class CodeGenerator : public AstNodeVisitor {
   // Add local variable descriptors to code.
   void FinalizeVarDescriptors(const Code& code) { UNIMPLEMENTED(); }
 
+  void FinalizeComments(const Code& code) { UNIMPLEMENTED(); }
+
   // Allocate and return an arguments descriptor.
   // Let 'num_names' be the length of 'optional_arguments_names'.
   // Treat the first 'num_arguments - num_names' arguments as positional and