Introduce simple Thread class to support gradual refactoring of interfaces.

runtime/vm/allocation.h

Index: runtime/vm/allocation.h
===================================================================
--- runtime/vm/allocation.h	(revision 43047)
+++ runtime/vm/allocation.h	(working copy)
@@ -93,6 +93,7 @@
   // Implicitly allocate the object in the current zone.
   void* operator new(uword size);
 
+  // DEPRECATED: Use Zone version.
   // Implicitly allocate the object in the current zone given the current
   // isolate.
   void* operator new(uword size, BaseIsolate* isolate);

[siva, 2015/01/21 22:14:50]

if deprecated would it make sense to move it under 'private:' and friend the few
classes that still use this version.

[koda, 2015/01/21 23:05:44]

There are still 10+ classes using it, at least one of which is an inner class,
which AFAIK cannot be forward-declared as a friend.

When the list is shorter and have no inner classes, I will do it.