UPGRADE FROM 3.x to 4.0
=======================
DependencyInjection
-------------------
* Using unsupported configuration keys in YAML configuration files raises an
exception.
* Using unsupported options to configure service aliases raises an exception.
Form
----
* The `choices_as_values` option of the `ChoiceType` has been removed.
Serializer
----------
* The ability to pass a Doctrine `Cache` instance to the `ClassMetadataFactory`
class has been removed. You should use the `CacheClassMetadataFactory` class
instead.
Yaml
* Removed support for passing `true`/`false` as the third argument to the `dump()` methods to toggle object support.
Before:
```php
Yaml::dump(array('foo' => new A(), 'bar' => 1), 0, 0, false, true);
```
After:
Yaml::dump(array('foo' => new A(), 'bar' => 1), 0, 0, false, Yaml::DUMP_OBJECT);
* The `!!php/object` tag to indicate dumped PHP objects was removed in favor of
the `!php/object` tag.