Check snapshots for feature compatibility.

runtime/vm/dart.h

Index: runtime/vm/dart.h
diff --git a/runtime/vm/dart.h b/runtime/vm/dart.h
index a21dab22176180fd6fcd3b650033c86115a7692c..ea40e43386e40197158ef7b83a4373fcb2e0f103 100644
--- a/runtime/vm/dart.h
+++ b/runtime/vm/dart.h
@@ -21,7 +21,7 @@ class ThreadPool;
 
 class Dart : public AllStatic {
  public:
-  static const char* InitOnce(
+  static char* InitOnce(
       const uint8_t* vm_isolate_snapshot,
       const uint8_t* instructions_snapshot,
       const uint8_t* data_snapshot,
@@ -61,6 +61,8 @@ class Dart : public AllStatic {
   static uword AllocateReadOnlyHandle();
   static bool IsReadOnlyHandle(uword address);
 
+  static const char* FeaturesString(Snapshot::Kind kind);
+
   static Snapshot::Kind snapshot_kind() {
     return snapshot_kind_;
   }