Created methods to surface zone memory information for each isolate and thread in JSON.

runtime/vm/zone.h

Index: runtime/vm/zone.h
diff --git a/runtime/vm/zone.h b/runtime/vm/zone.h
index 4e2e8a91e182a2f1cf8a012a687fc725ca816289..9cf29714e29bd2de0f39200fbfd78f485a5ed646 100644
--- a/runtime/vm/zone.h
+++ b/runtime/vm/zone.h
@@ -8,6 +8,7 @@
 #include "platform/utils.h"
 #include "vm/allocation.h"
 #include "vm/handles.h"
+#include "vm/json_stream.h"
 #include "vm/thread.h"
 #include "vm/memory_region.h"
 
@@ -65,6 +66,11 @@ class Zone {
   // due to internal fragmentation in the segments.
   intptr_t SizeInBytes() const;
 
+  // Computes the amount of space used in the zone.
+  intptr_t UsedSizeInBytes() const {
+    return SizeInBytes() - (limit_ - position_);
+  }
+
   // Structure for managing handles allocation.
   VMHandles* handles() { return &handles_; }
 
@@ -72,6 +78,10 @@ class Zone {
 
   Zone* previous() const { return previous_; }
 
+#ifndef PRODUCT
+  void PrintJSON(JSONStream* stream) const;
+#endif
+
  private:
   Zone();
   ~Zone();  // Delete all memory associated with the zone.
@@ -174,6 +184,9 @@ class StackZone : public StackResource {
   // due to internal fragmentation in the segments.
   intptr_t SizeInBytes() const { return zone_.SizeInBytes(); }
 
+  // Computes the used space in the zone.
+  intptr_t UsedSizeInBytes() const { return zone_.UsedSizeInBytes(); }
+
   Zone* GetZone() { return &zone_; }
 
  private: