ES6 symbols: symbol properties should not cause going into slow mode.

test/mjsunit/harmony/symbols.js

Index: test/mjsunit/harmony/symbols.js
diff --git a/test/mjsunit/harmony/symbols.js b/test/mjsunit/harmony/symbols.js
index 5e1515ee43cdf2cd2eb239c63648421454c34dea..c2002e6ac7298a24fa46d70a1ba57f06177d3905 100644
--- a/test/mjsunit/harmony/symbols.js
+++ b/test/mjsunit/harmony/symbols.js
@@ -213,9 +213,12 @@ TestCollections()
 
 
 function TestKeySet(obj) {
+  assertTrue(%HasFastProperties(obj))
   // Set the even symbols via assignment.
   for (var i = 0; i < symbols.length; i += 2) {
     obj[symbols[i]] = i
+    // Object should remain in fast mode until too many properties were added.
+    assertTrue(%HasFastProperties(obj) || i >= 30)
   }
 }