Disallow casting unquoted object literals to dict

Example:

``` js
/**
* @constructor
* @dict
*/
function Foo() {}
var x = /** @type {!Foo} */ ({ prop: 1 });
```

Discussion here:
https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/closure-compiler-discuss/k7IZqjh5_Hc/a45SwCV4BRMJ

I remember the casting rules were more liberal for object types, but I forget why now. @concavelenz do you remember?