Fix bug in `isWritable` utility function

Only attempt to re-set the property value in cases where it was
successfully modified as part of the function's execution. This avoids
errors when the underlying value is not writable. Rename the internal
result-tracking variable to make this more clear.

harness/propertyHelper.js

@@ -31,7 +31,7 @@ function isWritable(obj, name, verifyProp, value) {
     var newValue = value || "unlikelyValue";
     var hadValue = Object.prototype.hasOwnProperty.call(obj, name);
     var oldValue = obj[name];
-    var result;
+    var writeSucceeded;
 
     try {
         obj[name] = newValue;
@@ -41,16 +41,21 @@ function isWritable(obj, name, verifyProp, value) {
         }
     }
 
-    result = (verifyProp && isEqualTo(obj, verifyProp, newValue)) ||
+    writeSucceeded = (verifyProp && isEqualTo(obj, verifyProp, newValue)) ||
         isEqualTo(obj, name, newValue);
 
-    if (hadValue) {
-      obj[name] = oldValue;
-    } else {
-      delete obj[name];
+    // Revert the change only if it was successful (in other cases, reverting
+    // is unnecessary and may trigger exceptions for certain property
+    // configurations)
+    if (writeSucceeded) {
+      if (hadValue) {
+        obj[name] = oldValue;
+      } else {
+        delete obj[name];
+      }
     }
 
-    return result;
+    return writeSucceeded;
 }
 
 function verifyEqualTo(obj, name, value) {