Added dart:js samples landing pages.

Added dart:js samples landing pages.
Also fixed a couple of broken links.

R=kathyw@google.com

Committed: c31ec08

[shailentuli, 2013-11-05 23:15:13]

https://dartjs-dot-dart-lang.appspot.com/samples/

PTAL.

[shailentuli, 2013-11-06 12:42:42]

Staged version works now. Can you PTAL?

[Kathy Walrath, 2013-11-06 17:48:25]

lgtm

I should make a copyedit pass at some point. But here are a few issues I noticed
in files NOT in this CL:

https://code.google.com/p/dart/source/browse/branches/bleeding_edge/dart/samp...:

* Has wrong header. (Says ## Using jsonp with dart:js.)

* Explanation inconsistent with sample page: Doesn't mention Google Charts API.

* Remove "docs/releases/latest/" from API docs URL.


https://code.google.com/p/dart/source/browse/branches/bleeding_edge/dart/samp...:

* Github -> GitHub
  (I checked the capitalization on github.com.)

* Remove "docs/releases/latest/" from API docs URL.

https://chromiumcodereview.appspot.com/59753006/diff/40001/src/site/samples/j...
File src/site/samples/jsonp/index.markdown (right):

https://chromiumcodereview.appspot.com/59753006/diff/40001/src/site/samples/j...
src/site/samples/jsonp/index.markdown:3: title: Fetching data using JSONP
I thought we were using title case for titles?

https://chromiumcodereview.appspot.com/59753006/diff/40001/src/site/samples/j...
src/site/samples/jsonp/index.markdown:13: Github repos belonging to the user,
dart-lang.
Github -> GitHub

user, dart-lang -> user dart-lang

https://chromiumcodereview.appspot.com/59753006/diff/40001/src/site/samples/j...
src/site/samples/jsonp/index.markdown:16:
[source](https://code.google.com/p/dart/source/browse/#svn%2Fbranches%2Fbleeding_edge%2Fdart%2Fsamples%2Fjsonp);
; -> .

https://chromiumcodereview.appspot.com/59753006/diff/40001/src/site/samples/j...
src/site/samples/jsonp/index.markdown:23: See all [samples](/samples/)
. afterward?

(I don't think we're consistent about . vs. no .)

Actually, I'd be inclined to just have no period and full-line links. Like this:

[Read the source](...)
...
[See all samples](...)

[shailentuli, 2013-11-06 18:27:46]

Committed patchset #3 manually as rc31ec08 (presubmit successful).