Remember deopt-id -> context-level mappings in var descriptors.

runtime/vm/intermediate_language.h

Index: runtime/vm/intermediate_language.h
diff --git a/runtime/vm/intermediate_language.h b/runtime/vm/intermediate_language.h
index c4c8932832f053a386c95419ff56178635a8a4d5..588d082bc70a03551fc685729f18f6c3db2a823a 100644
--- a/runtime/vm/intermediate_language.h
+++ b/runtime/vm/intermediate_language.h
@@ -933,6 +933,8 @@ class Instruction : public ZoneAllocated {
   friend class Scheduler;
   friend class BlockEntryInstr;
   friend class CatchBlockEntryInstr;  // deopt_id_
+  friend class DebugStepCheckInstr;   // deopt_id_
+  friend class StrictCompareInstr;    // deopt_id_
 
   // Fetch deopt id without checking if this computation can deoptimize.
   intptr_t GetDeoptId() const { return deopt_id_; }
@@ -3628,8 +3630,12 @@ class NativeCallInstr : public TemplateDefinition<0, Throws> {
 
 class DebugStepCheckInstr : public TemplateInstruction<0, NoThrow> {
  public:
-  DebugStepCheckInstr(TokenPosition token_pos, RawPcDescriptors::Kind stub_kind)
-      : token_pos_(token_pos), stub_kind_(stub_kind) {}
+  DebugStepCheckInstr(TokenPosition token_pos,
+                      RawPcDescriptors::Kind stub_kind,
+                      intptr_t deopt_id)
+      : TemplateInstruction<0, NoThrow>(deopt_id),
+        token_pos_(token_pos),
+        stub_kind_(stub_kind) {}
 
   DECLARE_INSTRUCTION(DebugStepCheck)