[windows] Implements FreeSubSegment

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 25846136a996007789204103b8fc3aba808b33ed..f50d78fb718af3930a81929281180b8a797e39db 100644
--- a/runtime/vm/virtual_memory_win.cc
+++ b/runtime/vm/virtual_memory_win.cc
@@ -39,16 +39,16 @@ VirtualMemory::~VirtualMemory() {
     return;
   }
   if (VirtualFree(address(), 0, MEM_RELEASE) == 0) {
-    FATAL("VirtualFree failed");
+    FATAL1("VirtualFree failed: Error code %d\n", GetLastError());
   }
 }
 
 
 bool VirtualMemory::FreeSubSegment(void* address, intptr_t size) {
-  // On Windows only the entire segment returned by VirtualAlloc
-  // can be freed. Therefore we will have to waste these unused
-  // virtual memory sub-segments.
-  return false;
+  if (VirtualFree(address, size, MEM_DECOMMIT) == 0) {
+    FATAL1("VirtualFree failed: Error code %d\n", GetLastError());
+  }
+  return true;
 }