[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..3f17f3da88822a7a8301eec6116afa9cd1592dce 100644
--- a/runtime/vm/virtual_memory_win.cc
+++ b/runtime/vm/virtual_memory_win.cc
@@ -45,10 +45,10 @@ VirtualMemory::~VirtualMemory() {
 
 
 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) {
+    FATAL("VirtualFree failed");

[siva, 2016/10/13 16:11:40]

might be worthwhile to report the error code here so it is easier to figure out
what went wrong if this ever happens
FATAL1("VirtualFree failed : Error code %d\n", GetLastError());

[zra, 2016/10/13 16:56:08]

Done.

+  }
+  return true;
 }