Write protect executable pages in the VM.

runtime/vm/assembler.h

Index: runtime/vm/assembler.h
diff --git a/runtime/vm/assembler.h b/runtime/vm/assembler.h
index c08f923f5afe4911c2081f1513265a7747689926..043ea565e9e8872ebd7f90688680b3ad21140b2d 100644
--- a/runtime/vm/assembler.h
+++ b/runtime/vm/assembler.h
@@ -53,6 +53,8 @@ class AssemblerFixup : public ZoneAllocated {
  public:
   virtual void Process(const MemoryRegion& region, intptr_t position) = 0;
 
+  virtual bool IsPointerOffset() const = 0;
+
   // It would be ideal if the destructor method could be made private,
   // but the g++ compiler complains when this is subclassed.
   virtual ~AssemblerFixup() { UNREACHABLE(); }
@@ -118,6 +120,10 @@ class AssemblerBuffer : public ValueObject {
     fixup_ = fixup;
   }
 
+  // Count the fixups that produce a pointer offset, without processing
+  // the fixups.
+  intptr_t CountPointerOffsets() const;
+
   // Get the size of the emitted code.
   intptr_t Size() const { return cursor_ - contents_; }
   uword contents() const { return contents_; }