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

runtime/vm/heap.h

Index: runtime/vm/heap.h
diff --git a/runtime/vm/heap.h b/runtime/vm/heap.h
index 8feb555470ab0364177434828d2c6fdbf1ce5740..496a9c8ef049af3ffc2956ddc883a4a1a9536a60 100644
--- a/runtime/vm/heap.h
+++ b/runtime/vm/heap.h
@@ -133,7 +133,8 @@ class Heap {
   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);
@@ -358,6 +359,14 @@ class NoHeapGrowthControlScope : public StackResource {
   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_