Bug 1341061 - Test. (r=arai)

Will remove once https://github.com/tc39/test262/pull/869 merges and we
sync.

js/src/tests/test262/local/unscopables-inc-dec.js

@@ -0,0 +1,51 @@
+// Copyright (C) 2017 Mozilla Corporation. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+description:
+  @@unscopables should be looked up exactly once for inc/dec.
+info: |
+  UpdateExpression : LeftHandSideExpression ++
+  1. Let lhs be the result of evaluating LeftHandSideExpression.
+
+  GetIdentifierReference ( lex, name, strict )
+  [...]
+  3. Let exists be ? envRec.HasBinding(name).
+
+  HasBinding ( N )
+  [...]
+  6. Let unscopables be ? Get(bindings, @@unscopables).
+flags: [noStrict]
+features: [Symbol.unscopables]
+---*/
+
+var unscopablesGetterCalled = 0;
+var a, b, flag = true;
+with (a = { x: 7 }) {
+  with (b = { x: 4, get [Symbol.unscopables]() {
+                      unscopablesGetterCalled++;
+                      return { x: flag=!flag };
+                    } }) {
+    x++;
+  }
+}
+
+assert.sameValue(unscopablesGetterCalled, 1);
+assert.sameValue(a.x, 7);
+assert.sameValue(b.x, 5);
+
+flag = true;
+with (a = { x: 7 }) {
+  with (b = { x: 4, get [Symbol.unscopables]() {
+                      unscopablesGetterCalled++;
+                      return { x: flag=!flag };
+                    } }) {
+    x--;
+  }
+}
+
+assert.sameValue(unscopablesGetterCalled, 2);
+assert.sameValue(a.x, 7);
+assert.sameValue(b.x, 3);
+
+reportCompare(0,0);