Very first steps for a debugger

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
===================================================================
--- runtime/vm/isolate.h	(revision 1927)
+++ runtime/vm/isolate.h	(working copy)
@@ -18,6 +18,7 @@
 class ApiState;
 class BigintStore;
 class CodeIndexTable;
+class Debugger;
 class HandleScope;
 class Heap;
 class LongJump;
@@ -242,6 +243,8 @@
   intptr_t ast_node_id() const { return ast_node_id_; }
   void set_ast_node_id(int value) { ast_node_id_ = value; }
 
+  Debugger* debugger() const { return debugger_; }
+
  private:
   Isolate();
 
@@ -278,6 +281,7 @@
   ApiState* api_state_;
   StubCode* stub_code_;
   CodeIndexTable* code_index_table_;
+  Debugger* debugger_;
   LongJump* long_jump_base_;
   TimerList timer_list_;
   uword stack_limit_;