Basic tag infrastructure

runtime/vm/base_isolate.h

Index: runtime/vm/base_isolate.h
diff --git a/runtime/vm/base_isolate.h b/runtime/vm/base_isolate.h
index 5fb336d164b02b3e72d1723c35c6a935fd2d9be3..33d183683c3812a6bdea728286e3c4ed65d8ee64 100644
--- a/runtime/vm/base_isolate.h
+++ b/runtime/vm/base_isolate.h
@@ -98,6 +98,14 @@ class BaseIsolate {
   static void AssertCurrent(BaseIsolate* isolate);
 #endif
 
+  uword vm_tag() const {
+    return vm_tag_;
+  }
+
+  void set_vm_tag(uword tag) {
+    vm_tag_ = tag;
+  }
+
  protected:
   BaseIsolate()
       : top_resource_(NULL),
@@ -107,7 +115,8 @@ class BaseIsolate {
         no_handle_scope_depth_(0),
         no_gc_scope_depth_(0),
 #endif
-        no_callback_scope_depth_(0)
+        no_callback_scope_depth_(0),
+        vm_tag_(0)
   {}
 
   ~BaseIsolate() {
@@ -122,6 +131,7 @@ class BaseIsolate {
   int32_t no_gc_scope_depth_;
 #endif
   int32_t no_callback_scope_depth_;
+  uword vm_tag_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(BaseIsolate);