Enable polymorphic inlining of StringBase [] and StringBase codeUnitAt

runtime/vm/flow_graph_optimizer.h

Index: runtime/vm/flow_graph_optimizer.h
diff --git a/runtime/vm/flow_graph_optimizer.h b/runtime/vm/flow_graph_optimizer.h
index a3d035c5dc8668ca784f90636fc8b36a2a726bab..ecf6c224aa3bca70b503f0f12c0078a04b34f665 100644
--- a/runtime/vm/flow_graph_optimizer.h
+++ b/runtime/vm/flow_graph_optimizer.h
@@ -120,6 +120,22 @@ class FlowGraphOptimizer : public FlowGraphVisitor {
   void ReplaceWithInstanceOf(InstanceCallInstr* instr);
   void ReplaceWithTypeCast(InstanceCallInstr* instr);
 
+  Definition* PrepareInlineStringIndexOp(Instruction* call,
+                                         intptr_t cid,
+                                         Definition* str,
+                                         Definition* index,
+                                         Instruction* cursor);
+
+  bool InlineStringCodeUnitAt(Instruction* call,
+                              intptr_t cid,
+                              TargetEntryInstr** entry,
+                              Definition** last);
+
+  bool InlineStringBaseCharAt(Instruction* call,
+                              intptr_t cid,
+                              TargetEntryInstr** entry,
+                              Definition** last);
+
   LoadIndexedInstr* BuildStringCodeUnitAt(InstanceCallInstr* call,
                                           intptr_t cid);