Add Zone-based handle allocation interface and reduce use of Isolate-based interfaces.

runtime/vm/handles.cc

Index: runtime/vm/handles.cc
===================================================================
--- runtime/vm/handles.cc	(revision 43075)
+++ runtime/vm/handles.cc	(working copy)
@@ -43,17 +43,17 @@
 }
 
 
-uword VMHandles::AllocateHandle(Isolate* isolate) {
+uword VMHandles::AllocateHandle(Zone* zone) {
   return Handles<kVMHandleSizeInWords,
                  kVMHandlesPerChunk,
-                 kOffsetOfRawPtr>::AllocateHandle(isolate);
+                 kOffsetOfRawPtr>::AllocateHandle(zone);
 }
 
 
-uword VMHandles::AllocateZoneHandle(Isolate* isolate) {
+uword VMHandles::AllocateZoneHandle(Zone* zone) {
   return Handles<kVMHandleSizeInWords,
                  kVMHandlesPerChunk,
-                 kOffsetOfRawPtr>::AllocateZoneHandle(isolate);
+                 kOffsetOfRawPtr>::AllocateZoneHandle(zone);
 }