Cleanup app snapshots on isolate/vm exit.

runtime/bin/file_macos.cc

Index: runtime/bin/file_macos.cc
diff --git a/runtime/bin/file_macos.cc b/runtime/bin/file_macos.cc
index 842982f1dd98fa72909298613818b01c6f850037..f3d282941cd971f55fb28989387b52d757fadf6a 100644
--- a/runtime/bin/file_macos.cc
+++ b/runtime/bin/file_macos.cc
@@ -81,7 +81,7 @@ bool File::IsClosed() {
 }
 
 
-void* File::Map(MapType type, int64_t position, int64_t length) {
+MappedMemory* File::Map(MapType type, int64_t position, int64_t length) {
   ASSERT(handle_->fd() >= 0);
   int prot = PROT_NONE;
   switch (type) {
@@ -98,7 +98,15 @@ void* File::Map(MapType type, int64_t position, int64_t length) {
   if (addr == MAP_FAILED) {
     return NULL;
   }
-  return addr;
+  return new MappedMemory(addr, length);
+}
+
+
+void MappedMemory::Unmap() {
+  int result = munmap(address_, size_);
+  ASSERT(result == 0);
+  address_ = 0;
+  size_ = 0;
 }