Set a proper length reference to avoid conflict with browsers

In browsers length is a valid property of window, so they would have a normal
completion instead of a ReferenceError

src/dstr-binding/ary-ptrn-rest-obj-prop-id.case

@@ -18,6 +18,8 @@ info: |
        [...]
 ---*/
 
+//- setup
+let length = "outer";
 //- elems
 [...{ 0: v, 1: w, 2: x, 3: y, length: z }]
 //- vals
@@ -29,6 +31,4 @@ assert.sameValue(x, 9);
 assert.sameValue(y, undefined);
 assert.sameValue(z, 3);
 
-assert.throws(ReferenceError, function() {
-  length;
-});
+assert.sameValue(length, "outer", "the length prop is not set as a binding name");