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

runtime/vm/flow_graph_compiler_x64.cc

Index: runtime/vm/flow_graph_compiler_x64.cc
diff --git a/runtime/vm/flow_graph_compiler_x64.cc b/runtime/vm/flow_graph_compiler_x64.cc
index d923279dbc55e077f5713fe97aa590a90d1cd74c..976c35b5cde02499bdfd1c1e501840997f507e13 100644
--- a/runtime/vm/flow_graph_compiler_x64.cc
+++ b/runtime/vm/flow_graph_compiler_x64.cc
@@ -1122,11 +1122,13 @@ void FlowGraphCompiler::VisitCatchEntry(CatchEntryComp* comp) {
 
 void FlowGraphCompiler::VisitBlocks() {
   for (intptr_t i = 0; i < block_order_.length(); ++i) {
+    __ Comment("B%d", i);
     // Compile the block entry.
     current_block_ = block_order_[i];
     Instruction* instr = current_block()->Accept(this);
     // Compile all successors until an exit, branch, or a block entry.
     while ((instr != NULL) && !instr->IsBlockEntry()) {
+      __ Comment("@%d: instruction", instr->cid());
       instr = instr->Accept(this);
     }
 
@@ -1701,6 +1703,11 @@ void FlowGraphCompiler::FinalizeExceptionHandlers(const Code& code) {
 }
 
 
+void FlowGraphCompiler::FinalizeComments(const Code& code) {
+  code.set_comments(assembler_->GetCodeComments());
+}
+
+
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64