Cache isolate in the StackTrace object to that we don't call

runtime/vm/allocation.h

Index: runtime/vm/allocation.h
===================================================================
--- runtime/vm/allocation.h	(revision 1423)
+++ runtime/vm/allocation.h	(working copy)
@@ -9,6 +9,9 @@
 
 namespace dart {
 
+// Forward declarations.
+class Isolate;
+
 // Stack allocated objects subclass from this base class. Objects of this type
 // cannot be allocated on either the C or object heaps. Destructors for objects
 // of this type will not be run unless the stack is unwound through normal
@@ -35,11 +38,14 @@
   StackResource();
   virtual ~StackResource();
 
+  Isolate* isolate() const { return isolate_; }
+
   // The delete operator should be private instead of public, but unfortunately
   // the compiler complains when compiling the destructors for subclasses.
   void operator delete(void* pointer) { UNREACHABLE(); }
 
  private:
+  Isolate* isolate_;  // Current isolate for this stack resource.
   StackResource* previous_;
 
   void* operator new(uword size);