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

runtime/vm/kernel_to_il.h

Index: runtime/vm/kernel_to_il.h
diff --git a/runtime/vm/kernel_to_il.h b/runtime/vm/kernel_to_il.h
index cc3f0069abbbac382eb681a24ee54029391fca9b..7663fd393cc7e234236583bc296549a305d5a19f 100644
--- a/runtime/vm/kernel_to_il.h
+++ b/runtime/vm/kernel_to_il.h
@@ -786,6 +786,7 @@ class FlowGraphBuilder : public ExpressionVisitor, public StatementVisitor {
   FlowGraphBuilder(TreeNode* node,
                    ParsedFunction* parsed_function,
                    const ZoneGrowableArray<const ICData*>& ic_data_array,
+                   ZoneGrowableArray<intptr_t>* context_level_array,
                    InlineExitCollector* exit_collector,
                    intptr_t osr_id,
                    intptr_t first_block_id = 1);
@@ -1052,14 +1053,21 @@ class FlowGraphBuilder : public ExpressionVisitor, public StatementVisitor {
   ParsedFunction* parsed_function_;
   intptr_t osr_id_;
   const ZoneGrowableArray<const ICData*>& ic_data_array_;
+  // Contains (deopt_id, context_level) pairs.
+  ZoneGrowableArray<intptr_t>* context_level_array_;
   InlineExitCollector* exit_collector_;
 
   intptr_t next_block_id_;
   intptr_t AllocateBlockId() { return next_block_id_++; }
 
   intptr_t GetNextDeoptId() {
-    // TODO(rmacnak): Record current scope / context level.
-    return thread_->GetNextDeoptId();
+    intptr_t deopt_id = thread_->GetNextDeoptId();
+    if (context_level_array_ != NULL) {
+      intptr_t level = context_depth_;
+      context_level_array_->Add(deopt_id);
+      context_level_array_->Add(level);
+    }
+    return deopt_id;
   }
 
   intptr_t next_function_id_;