Revert r4782. Fix issues 728, 732.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index fe33e7ef5110a8ac6aa57e086cc0e5cbb2c603a1..0dc5d00983c5982459e4de6ea5e93b972aec4d91 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -2986,8 +2986,7 @@ StringHasher::StringHasher(int length)
   : length_(length),
     raw_running_hash_(0),
     array_index_(0),
-    is_array_index_(0 < length_ &&
-                    length_ <= String::kMaxCachedArrayIndexLength),
+    is_array_index_(0 < length_ && length_ <= String::kMaxArrayIndexSize),
     is_first_char_(true),
     is_valid_(true) { }
 
@@ -3050,7 +3049,9 @@ uint32_t StringHasher::GetHash() {
 
 bool String::AsArrayIndex(uint32_t* index) {
   uint32_t field = hash_field();
-  if (IsHashFieldComputed(field) && !(field & kIsArrayIndexMask)) return false;
+  if (IsHashFieldComputed(field) && (field & kIsNotArrayIndexMask)) {
+    return false;
+  }
   return SlowAsArrayIndex(index);
 }