Double dict unpacking does not parse

```
$ python -c "a = {'a': 1}
b = {'b': 2}
print(dict(**a, **b))
"
{'a': 1, 'b': 2}
```

```
$ mypy -c "a = {'a': 1}
b = {'b': 2}
print(dict(**a, **b))
"
<string>:3: error: Parse error before **
```


```
$ python --version
Python 3.5.2
```
```
$ mypy --version
mypy 0.4.5
```