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

runtime/vm/flow_graph_builder.h

Index: runtime/vm/flow_graph_builder.h
diff --git a/runtime/vm/flow_graph_builder.h b/runtime/vm/flow_graph_builder.h
index 5fbc056f2966357e24e1e47cc8f956a3a1300eef..b4018dfdc8a4b937b6759363d053140a7795de4d 100644
--- a/runtime/vm/flow_graph_builder.h
+++ b/runtime/vm/flow_graph_builder.h
@@ -98,6 +98,7 @@ class FlowGraphBuilder : public ValueObject {
   // id of the OSR entry or Compiler::kNoOSRDeoptId if not compiling for OSR.
   FlowGraphBuilder(const ParsedFunction& parsed_function,
                    const ZoneGrowableArray<const ICData*>& ic_data_array,
+                   ZoneGrowableArray<intptr_t>* context_level_array,
                    InlineExitCollector* exit_collector,
                    intptr_t osr_id);
 
@@ -114,10 +115,7 @@ class FlowGraphBuilder : public ValueObject {
   intptr_t AllocateBlockId() { return ++last_used_block_id_; }
   void SetInitialBlockId(intptr_t id) { last_used_block_id_ = id; }
 
-  intptr_t GetNextDeoptId() {
-    // TODO(rmacnak): Record current scope / context level.
-    return thread()->GetNextDeoptId();
-  }
+  intptr_t GetNextDeoptId() const;
 
   intptr_t context_level() const;
 
@@ -198,6 +196,8 @@ class FlowGraphBuilder : public ValueObject {
 
   const ParsedFunction& parsed_function_;
   const ZoneGrowableArray<const ICData*>& ic_data_array_;
+  // Contains (deopt_id, context_level) pairs.
+  ZoneGrowableArray<intptr_t>* context_level_array_;
 
   const intptr_t num_copied_params_;
   const intptr_t num_non_copied_params_;