Object.observe: prevent observed objects from using fast elements.

test/mjsunit/regress/regress-observe-empty-double-array.js

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@@ skipping 14 matching lines @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 // Flags: --harmony-observation --allow-natives-syntax
 //
 // Test passes if it does not crash.
 
 arr = [1.1];
 Object.observe(arr, function(){});
 arr.length = 0;
-assertTrue(%HasFastDoubleElements(arr));
+// TODO(observe): we currently disallow fast elements for observed object.
+// assertTrue(%HasFastDoubleElements(arr));
 // Should not crash
 arr.push(1.1);