Changed StackZone and ApiZone to be containers for Zone.

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index 9c2d491952230289c1beddc32e48b6cf756ad1d1..7a0351254b820ff92a16e40de2a725f7baad8997 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -9709,7 +9709,7 @@ RawBigint* Bigint::Allocate(intptr_t length, Heap::Space space) {
 
 
 static uword BigintAllocator(intptr_t size) {
-  StackZone* zone = Isolate::Current()->current_zone();
+  Zone* zone = Isolate::Current()->current_zone();
   return zone->AllocUnsafe(size);
 }
 
@@ -10221,7 +10221,7 @@ RawString* String::NewFormattedV(const char* format, va_list args) {
   intptr_t len = OS::VSNPrint(NULL, 0, format, args_copy);
   va_end(args_copy);
 
-  StackZone* zone = Isolate::Current()->current_zone();
+  Zone* zone = Isolate::Current()->current_zone();
   char* buffer = zone->Alloc<char>(len + 1);
   OS::VSNPrint(buffer, (len + 1), format, args);
 
@@ -10326,7 +10326,7 @@ RawString* String::SubString(const String& str,
 
 const char* String::ToCString() const {
   intptr_t len = Utf8::Length(*this);
-  StackZone* zone = Isolate::Current()->current_zone();
+  Zone* zone = Isolate::Current()->current_zone();
   char* result = zone->Alloc<char>(len + 1);
   Utf8::Encode(*this, result, len);
   result[len] = 0;