Clarify heap write protection interface and use WritableVMIsolateScope to resolve issue 23217.

runtime/vm/heap.h

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_HEAP_H_
 #define VM_HEAP_H_
 
 #include "platform/assert.h"
 #include "vm/allocation.h"
 #include "vm/flags.h"
@@ skipping 115 matching lines @@
   void CollectGarbage(Space space, ApiCallbacks api_callbacks, GCReason reason);
   void CollectAllGarbage();
 
   // Enables growth control on the page space heaps.  This should be
   // called before any user code is executed.
   void EnableGrowthControl() { SetGrowthControlState(true); }
   void DisableGrowthControl() { SetGrowthControlState(false); }
   void SetGrowthControlState(bool state);
   bool GrowthControlState();
 
-  // Protect access to the heap.
+  // Protect access to the heap. Note: Code pages are made
+  // executable/non-executable when 'read_only' is true/false, respectively.
   void WriteProtect(bool read_only);
   void WriteProtectCode(bool read_only) {
     old_space_.WriteProtectCode(read_only);
   }
 
   // Accessors for inlined allocation in generated code.
   static intptr_t TopOffset(Space space);
   static intptr_t EndOffset(Space space);
   static Space SpaceForAllocation(intptr_t class_id);
 
@@ skipping 204 matching lines @@
 
 class NoHeapGrowthControlScope : public StackResource {
  public:
   NoHeapGrowthControlScope();
   ~NoHeapGrowthControlScope();
  private:
   bool current_growth_controller_state_;
   DISALLOW_COPY_AND_ASSIGN(NoHeapGrowthControlScope);
 };
 
+
+// Note: During this scope, the code pages are non-executable.
+class WritableVMIsolateScope : StackResource {
+ public:
+  explicit WritableVMIsolateScope(Thread* thread);
+  ~WritableVMIsolateScope();
+};
+
 }  // namespace dart
 
 #endif  // VM_HEAP_H_