dartdoc should not use computeNode()

[jcollins-g]

@scheglov Konstantin, can you explain more about why using computeNode() is a bad idea? Dartdoc's instances of it seemed fairly benign at first glance to me.

[scheglov]

I don't remember the clincher, but here are some idea.
@bwilkerson might remember more ideas.

1. There is no way for computeNode() to guarantee that it will read the same content of a file as the one which was used to create the element.

2. When you call computeNode(), it has to perform full resolution, even if the client does not care about resolution of the full file, with method bodies.

3. It's a bad API - it mixes elements and AST nodes.

[srawlins]

Yay computeNode is no longer referenced or overridden.