Landing: Write protect executable pages in the VM.

runtime/vm/code_patcher.h

Index: runtime/vm/code_patcher.h
===================================================================
--- runtime/vm/code_patcher.h	(revision 31654)
+++ runtime/vm/code_patcher.h	(working copy)
@@ -22,6 +22,17 @@
 class RawObject;
 class String;
 
+class WritableInstructionsScope : public ValueObject {
+ public:
+  WritableInstructionsScope(uword address, intptr_t size);
+  ~WritableInstructionsScope();
+
+ private:
+  uword address_;
+  intptr_t size_;
+};
+
+
 class CodePatcher : public AllStatic {
  public:
   // Dart static calls have a distinct, machine-dependent code pattern.