Default export for a function should be a function expression.

In this case, the function may not have a name hence the syntax node
should not be a function declaration.

https://code.google.com/p/esprima/issues/detail?id=410
Tolerate missing yield in a generator.

https://code.google.com/p/esprima/issues/detail?id=431
Make sure default parameters are also available in function property.

https://code.google.com/p/esprima/issues/detail?id=432
Support default parameters for concise method definition.

https://code.google.com/p/esprima/issues/detail?id=432
Support default parameters for arrow function.

https://code.google.com/p/esprima/issues/detail?id=432
Contextualize 'module' as the keyword.

Since we don't distinguish between parsing a module or not, the decision
for a 'module' keyword is based on the subsequent token (module
declaration is only followed by a string literal or another identifier).

https://code.google.com/p/esprima/issues/detail?id=443