[stubs] Properly handle read-only properties in StoreGlobalViaContextStub.

test/mjsunit/regress/cross-script-vars.js

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Flags: --allow-natives-syntax
 
 function PrintDesc(desc, s) {
   var json;
   if (desc) {
     json = JSON.stringify(desc);
@@ skipping 434 matching lines @@
     assertEquals(true, GetDescriptor().configurable);
     assertEquals(false, GetDescriptor().writable);
 
     assertEquals(42, LoadVar());
     assertEquals(true, GetDescriptor().configurable);
     assertThrows('DefineVar(153)');
     assertEquals(42, LoadVar());
     assertThrows('StoreVar(113)');
     assertThrows('StoreVar(113)');
     assertEquals(42, LoadVar());
+    assertThrows('StoreVar(42)');
+    assertEquals(42, LoadVar());
     assertThrows('LoadStoreLoop()');
     assertEquals(42, LoadVar());
     TearDown();
   })();
 
 
   (function() {
     SetUp();
     cfg = {optimize: opt_cfg.optimize, strict: true};
     ForceMutablePropertyCellType();
@@ skipping 99 matching lines @@
     assertEquals(0, counters.set_count);
 
     TearDown();
   })();
 
 }  // testSuite
 
 
 testSuite({optimize: false});
 testSuite({optimize: true});