Any identifier with two pairs of consecutive dots is interpreted as calculated aesthetics

Failing:

```
ggplot() + geom_histogram(aes(x=runif(1000), y=xxx..density..yyy))
## stat_bin: binwidth defaulted to range/30. Use 'binwidth = x' to adjust this.
## Error in eval(expr, envir, enclos) : object 'xxxdensityyyy' not found
```

Succeeding:

```
ggplot() + geom_histogram(aes(x=runif(1000), y=d..ensit..y))
```

Not a real problem, but one with a trivial fix. My favorite :-)
