Avoid trusting the length encoded in the Snapshot if there is an

runtime/vm/dart_api_message.cc

Index: runtime/vm/dart_api_message.cc
===================================================================
--- runtime/vm/dart_api_message.cc	(revision 10993)
+++ runtime/vm/dart_api_message.cc	(working copy)
@@ -11,10 +11,8 @@
 
 static const int kNumInitialReferences = 4;
 
-ApiMessageReader::ApiMessageReader(const uint8_t* buffer,
-                                   intptr_t length,
-                                   ReAlloc alloc)
-    : BaseReader(buffer, length),
+ApiMessageReader::ApiMessageReader(const Snapshot* snapshot, ReAlloc alloc)
+    : BaseReader(snapshot->content(), snapshot->length()),
       alloc_(alloc),
       backward_references_(kNumInitialReferences) {
   Init();
@@ -460,7 +458,7 @@
 }
 
 
-void ApiMessageWriter::WriteMessage(intptr_t field_count, intptr_t *data) {
+intptr_t ApiMessageWriter::WriteMessage(intptr_t field_count, intptr_t *data) {
   // Write out the serialization header value for this object.
   WriteInlinedObjectHeader(kMaxPredefinedObjectIds);
 
@@ -479,7 +477,7 @@
     Write<RawObject*>(Integer::New(data[i]));
   }
 
-  FinalizeBuffer();
+  return FinalizeBuffer();
 }
 
 
@@ -773,7 +771,7 @@
 }
 
 
-void ApiMessageWriter::WriteCMessage(Dart_CObject* object) {
+intptr_t ApiMessageWriter::WriteCMessage(Dart_CObject* object) {
   WriteCObject(object);
   // Write out all objects that were added to the forward list and have
   // not been serialized yet. These would typically be fields of arrays.
@@ -782,7 +780,7 @@
     WriteForwardedCObject(forward_list_[i]);
   }
   UnmarkAllCObjects(object);
-  FinalizeBuffer();
+  return FinalizeBuffer();
 }
 
 }  // namespace dart