Allows turning on checked mode on a per-isolate basis

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
===================================================================
--- runtime/vm/isolate.h	(revision 43426)
+++ runtime/vm/isolate.h	(working copy)
@@ -357,6 +357,9 @@
     return OFFSET_OF(Isolate, single_step_);
   }
 
+  void set_checked_mode(bool value) { checked_mode_ = value; }
+  bool checked_mode() const { return checked_mode_; }
+
   // Requests that the debugger resume execution.
   void Resume() {
     resume_request_ = true;
@@ -649,6 +652,7 @@
   Debugger* debugger_;
   bool single_step_;
   bool resume_request_;
+  bool checked_mode_;
   Random random_;
   Simulator* simulator_;
   LongJumpScope* long_jump_base_;