Fix clang builds.

runtime/vm/become.cc

Index: runtime/vm/become.cc
diff --git a/runtime/vm/become.cc b/runtime/vm/become.cc
index a037f9d8b846a6ac46dee457ad94a1c4d4190412..accca31f396de7ee188a33f5222d30890d99b211 100644
--- a/runtime/vm/become.cc
+++ b/runtime/vm/become.cc
@@ -152,27 +152,31 @@ class ForwardHeapPointersHandleVisitor : public HandleVisitor {
 // which is normally write-protected, so we need to make it temporarily writable
 // to forward the pointers. On all other architectures, object pointers are
 // accessed through ObjectPools.
+#if defined(TARGET_ARCH_IA32)
 class WritableCodeLiteralsScope : public ValueObject {
  public:
   explicit WritableCodeLiteralsScope(Heap* heap) : heap_(heap) {
-#if defined(TARGET_ARCH_IA32)
     if (FLAG_write_protect_code) {
       heap_->WriteProtectCode(false);
     }
-#endif
   }
 
   ~WritableCodeLiteralsScope() {
-#if defined(TARGET_ARCH_IA32)
     if (FLAG_write_protect_code) {
       heap_->WriteProtectCode(true);
     }
-#endif
   }
 
  private:
   Heap* heap_;
 };
+#else
+class WritableCodeLiteralsScope : public ValueObject {
+ public:
+  explicit WritableCodeLiteralsScope(Heap* heap) { }
+  ~WritableCodeLiteralsScope() { }
+};
+#endif
 
 
 void Become::MakeDummyObject(const Instance& instance) {