Write protect executable pages in the VM.

runtime/vm/virtual_memory_win.cc

Index: runtime/vm/virtual_memory_win.cc
diff --git a/runtime/vm/virtual_memory_win.cc b/runtime/vm/virtual_memory_win.cc
index d1a9277741ad6bfaf55ac8364ed74158f6c3ec19..e1c7e498bf6498e672e8ea7cc46af67e088f4058 100644
--- a/runtime/vm/virtual_memory_win.cc
+++ b/runtime/vm/virtual_memory_win.cc
@@ -63,7 +63,10 @@ bool VirtualMemory::Commit(uword addr, intptr_t size, bool executable) {
 }
 
 
-bool VirtualMemory::Protect(Protection mode) {
+bool VirtualMemory::Protect(void* address, intptr_t size, Protection mode) {
+  uword start_address = reinterpret_cast<uword>(address);
+  uword end_address = start_address + size;
+  uword page_address = start_address & ~(PageSize() - 1);
   DWORD prot = 0;
   switch (mode) {
     case kNoAccess:
@@ -83,7 +86,10 @@ bool VirtualMemory::Protect(Protection mode) {
       break;
   }
   DWORD old_prot = 0;
-  return VirtualProtect(address(), size(), prot, &old_prot);
+  return VirtualProtect(reinterpret_cast<void*>(address),
+                        end_address - page_address,
+                        prot,
+                        &old_prot);
 }
 
 }  // namespace dart