Give a length field to stack bitmaps.

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index af8695abcf43b802c1adeeeed8abc8553a5c65a3..4bd7cf57866aab75d56a900ea8fbb4850d5c0f13 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -6773,13 +6773,12 @@ void Stackmap::SetBit(intptr_t bit_index, bool value) const {
 }
 
 
-RawStackmap* Stackmap::New(intptr_t pc_offset,
-                           intptr_t length,
-                           BitmapBuilder* bmap) {
+RawStackmap* Stackmap::New(intptr_t pc_offset, BitmapBuilder* bmap) {
   ASSERT(Object::stackmap_class() != Class::null());
   ASSERT(bmap != NULL);
   Stackmap& result = Stackmap::Handle();
   // Guard against integer overflow of the instance size computation.
+  intptr_t length = bmap->Length();
   intptr_t payload_size =
       Utils::RoundUp(length, kBitsPerByte) / kBitsPerByte;
   if (payload_size < 0 ||
@@ -6807,7 +6806,6 @@ RawStackmap* Stackmap::New(intptr_t pc_offset,
   for (intptr_t i = 0; i < length; ++i) {
     result.SetBit(i, bmap->Get(i));
   }
-  ASSERT(bmap->Maximum() < length);
   return result.raw();
 }