dartdoc should group extension method with the class being extended if they are in the same import

[brianquinlan]

Motivation:

1. I want to know more about Futures
2. I google for "Future dart" and end up on the Future<T> documentation page
3. I miss the existence of the ignore method because it is documented separately in the Extensions section

In this case, ignore is implemented as an extension method for compatibility reasons (adding a method to an existing class can break 3p code that implements that class) not because it is logically separate from the class.

Ideally, if the extension method is defined in the same package, it would be documented with the class or, at minimum, a link to the related extensions should be shown.

[srawlins]

+1 this is a great idea.

[srawlins]

Duplicate of #2021