Implement constructor tear-offs in dartdoc

[jcollins-g]

Depends on dart-lang/sdk#46020.

We can add to dartdoc's internal reference scheme for unnamed constructor tearoffs based on the specification, here: https://github.com/dart-lang/language/blob/master/accepted/future-releases/constructor-tearoffs/feature-specification.md. Specifically, allowing comment references to use [C.new] for unnamed constructor tearoffs will simplify some things in the new reference lookup code.

[Edit by eernstg on Jul 8th: Updated link to feature spec.]

[jcollins-g]

Unblocking as it should be possible to begin development, and not crash for simple uses now.

[jcollins-g]

Now looking at three major features for constructor tearoffs in dartdoc:

• Rendering the values of constants when they are assigned a constructor tearoff
• Correct linkage of .new when referring to default constructors of classes
• Correct linkage of .new and other constructors when referring to typedefs, as appropriate

[devoncarew]

@jcollins-g - is this generally on track for early October?

[jcollins-g]

yes.