[es6] Partially implement Reflect.preventExtensions.

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index 8f4915cefd55bbab5f0f19b70a41ad7a08a8e734..04c00ba0c873edf4a0b4c91b8e6fefb813be67b8 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -164,6 +164,19 @@ typedef ZoneList<Handle<Object> > ZoneObjectList;
   RETURN_ON_EXCEPTION_VALUE(isolate, call, MaybeHandle<T>())
 
 
+// Macros for Maybe.
+
+#define MAYBE_ASSIGN_RETURN_ON_EXCEPTION(isolate, T, dst, call) \
+  do {                                                          \
+    Maybe<T> result_of_the_call = (call);                       \
+    if (result_of_the_call.IsNothing()) {                       \
+      DCHECK((isolate)->has_pending_exception());               \
+      return isolate->heap()->exception();                      \
+    }                                                           \
+    (dst) = result_of_the_call.FromJust();                      \
+  } while (false)
+
+
 #define FOR_EACH_ISOLATE_ADDRESS_NAME(C)                \
   C(Handler, handler)                                   \
   C(CEntryFP, c_entry_fp)                               \