[Dart] follow-up to #2761 - fix broken links and transform API links for dart docs

[chalin]

Follow-up to #2761, where @kwalrath wrote #2761 (comment):

• Link to InjectableMetadata goes to TS... and is a 404: https://angular.io/docs/ts/latest/api/core/index/InjectableMetadata-class.html (InjectableMetadata no longer exists in Dart) - docs: intra-site links should be relative #2773
• Links to viewChild and contentChild should be ViewChild and ContentChild in the text.
• I ran the script, and it seemed to work although it didn't catch the bad API link in dependency-injection.html - docs: intra-site links should be relative #2773

[chalin]

@kwalrath - InjectableMetadata link is fixed via #2773 (where is was also changed to refer to just Injectable). The reason the checker didn't pick up on that bad link is because it was an absolute link and we're only applying transformations to relative links. (The reasoning is that if the author went to the trouble of typing an absolute link then we assume it is correct, and not a candidate for transformation ... even if it refers to some API page).

[chalin]

@kwalrath - I fixed the capitalization of ViewChild and ContentChild in a new commit, named "Kathy's post-review edits 2", to each of #2750 and #2751.

[chalin]

All issue items have been addressed.