Incorrect handling of nullable types defined in typedefs

The following code will compile, but it should not.

```
/**
* @param {!FooBar} foo
*/
function say(foo) {
};

/**
* @typedef {?{
* name: string
* }}
*/
var FooBar;
```

This bug is very bad, as it makes us believe that FooBar is non-nullable, but it's not true. In Chromium we use this syntax a lot for defining new types in externs files, and we have possibly a lot of bugs which are not caught by the Closure Compiler.