conditional based on parameter undefined/defined fail when value is parameter

Nobody has claimed this yet.

If I reference a value that is undefined in the current context, I get an unwelcome warning:
e.g., with parent context
```
{
data: [{
item : "exampleA",
blah : "This example"
},{
item : "exampleB"
}]
}
```
And a template "boo.dust"
```
{blah} is good
```

Invoked as a section:
```
{#data}
{> boo/}
{/data}
```
The second entry, exampleB, gives a warning:

> [DUST:INFO] Cannot find reference '{blah}' in template 'boo'

What I need to do is conditionally render a partial like this:
```
{@exists key='blah'}
{blah} is good
{/exists}
```

I wrote a helper that works fine in the above example, but it fails if instead of passing the key in the context, I send it as a parameter, e.g.,
**FAIL**
```
{
data: [{
item : "exampleA",
},{
item : "exampleB"
}]
}
```
And a template "boo.dust"
```
{blah} is good
```

Invoked as a section:
```
{#data}
{> boo blah="THIS EXAMPLE"/}
{/data}
```

I tried using {?exists/} as described in http://www.dustjs.com/docs/syntax/ but there isn't any actual example code, so it's not clear how to do it and I couldn't get it working.

Any pointers welcome.

Start with the `boo.dust` examples and the documented `{?exists/}` syntax, then reproduce the difference between a context key and the partial parameter `blah="THIS EXAMPLE"`. Trace how Dust resolves the parameter inside the `data` section and verify whether the conditional can render without an undefined-reference warning.

Written by the indexing model from the issue text.