yapf gets confused when a dict literal has lambdas as values

Input:
```
{
'a': lambda a, b: True,
'b': lambda c, d: False,
}
```

yapf wants to apply this diff:

```
--- narf.py (original)
+++ narf.py (reformatted)
@@ -1,4 +1,6 @@
{
- 'a': lambda a, b: True,
- 'b': lambda c, d: False,
+ 'a': lambda a,
+ b: True,
+ 'b': lambda c,
+ d: False,
}
```

IOW

```
{
'a': lambda a,
b: True,
'b': lambda c,
d: False,
}
```

Which strikes me as awful. This is with `allow_multiline_lambdas = true` in case that matters. Noticed on the hg codebase, where we're considering using yapf.