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

runtime/vm/dart.cc

Index: runtime/vm/dart.cc
diff --git a/runtime/vm/dart.cc b/runtime/vm/dart.cc
index 47b4221fda26c4d284c1c24613288ef790e5c383..e37497ce5dece16b2b27c585c35f9a44d7d35755 100644
--- a/runtime/vm/dart.cc
+++ b/runtime/vm/dart.cc
@@ -668,6 +668,10 @@ const char* Dart::FeaturesString(Isolate* isolate, Snapshot::Kind kind) {
 #endif
 
   if (Snapshot::IncludesCode(kind)) {
+    if (FLAG_support_debugger) {
+      buffer.AddString(" support-debugger");
+    }
+
     // Checked mode affects deopt ids.
 #define ADD_FLAG(name, isolate_flag, flag)                                     \
   do {                                                                         \