Parsing support (including incremental parsing) for 'yield' expressions.

Yes, though it took me a while to walk through this all (it's unpleasant that there's no grammar we can just look at at this point).  

Here's the chain of productions through which we can see the 'GeneratorParameter' being passed down:

```
GeneratorDeclaration[Yield, Default] :
  function * BindingIdentifier[?Yield] ( FormalParameters[Yield,GeneratorParameter] ) { GeneratorBody[Yield] }
  ... elided ...

// FormalParameters[yield,generateparameter] leads to:
FormalParameter[Yield,GeneratorParameter] :
  BindingElement[?Yield, ?GeneratorParameter]

BindingElement[Yield, GeneratorParameter ] : See 13.2.3
  ... elided ...
  [+GeneratorParameter] BindingPattern[?Yield,GeneratorParameter] Initializer[In]opt

BindingPattern[Yield,GeneratorParameter] :
  ObjectBindingPattern[?Yield,?GeneratorParameter]
  ... elided ...

ObjectBindingPattern[Yield,GeneratorParameter] :
  ... elided ...
  { BindingPropertyList[?Yield,?GeneratorParameter] }

BindingPropertyList[Yield,GeneratorParameter] :
  BindingProperty[?Yield, ?GeneratorParameter]
  ... elided ...

BindingProperty[Yield,GeneratorParameter] :
  ... elided ...
  PropertyName[?Yield, ?GeneratorParameter] : BindingElement[?Yield, ?GeneratorParameter]
```