Write the magic number 0xf5f5dcdc into a script snapshot file and use this to distinguish whether a…

runtime/vm/dart_api_impl.cc

Index: runtime/vm/dart_api_impl.cc
===================================================================
--- runtime/vm/dart_api_impl.cc	(revision 20286)
+++ runtime/vm/dart_api_impl.cc	(working copy)
@@ -4292,7 +4292,8 @@
 }
 
 
-DART_EXPORT Dart_Handle Dart_LoadScriptFromSnapshot(const uint8_t* buffer) {
+DART_EXPORT Dart_Handle Dart_LoadScriptFromSnapshot(const uint8_t* buffer,
+                                                    intptr_t buffer_len) {
   Isolate* isolate = Isolate::Current();
   DARTSCOPE(isolate);
   TIMERSCOPE(time_script_loading);
@@ -4304,6 +4305,11 @@
     return Api::NewError("%s expects parameter 'buffer' to be a script type"
                          " snapshot.", CURRENT_FUNC);
   }
+  if (snapshot->length() != buffer_len) {
+    return Api::NewError("%s: 'buffer_len' of %"Pd" is not equal to %d which"
+                         " is the expected length in the snapshot.",
+                         CURRENT_FUNC, buffer_len, snapshot->length());
+  }
   Library& library =
       Library::Handle(isolate, isolate->object_store()->root_library());
   if (!library.IsNull()) {