Add some dartdoc comments

Add some dartdoc comments

This CL is an experiment in adding some dartdoc comments in a few places. It's
unclear how extensively we'll use dartdoc.

TBR=ianh@google.com

Committed: https://chromium.googlesource.com/external/mojo/+/c0a1b3bc363413a15f2a6247521168c2a9c25331

[abarth-chromium, 2015-07-08 04:39:30]

Committed patchset #1 (id:1) manually as
c0a1b3bc363413a15f2a6247521168c2a9c25331 (presubmit successful).

[Hixie, 2015-07-08 15:10:57]

lgtm

https://codereview.chromium.org/1207373005/diff/1/sky/sdk/lib/widgets/dialog....
File sky/sdk/lib/widgets/dialog.dart (right):

https://codereview.chromium.org/1207373005/diff/1/sky/sdk/lib/widgets/dialog....
sky/sdk/lib/widgets/dialog.dart:23: /// The (optional) title of the dialog is
displayed in a large font at the top
This is an interesting convention (putting the documentation on the fields,
rather than on the constructor). What does the output look like? Is it clear
that these really refer to the constructor arguments?

If we document these, we're gonna have to reconsider making the (mutable) fields
public on StatefulComponents.

[abarth-chromium, 2015-07-08 15:29:29]

https://codereview.chromium.org/1207373005/diff/1/sky/sdk/lib/widgets/dialog....
File sky/sdk/lib/widgets/dialog.dart (right):

https://codereview.chromium.org/1207373005/diff/1/sky/sdk/lib/widgets/dialog....
sky/sdk/lib/widgets/dialog.dart:23: /// The (optional) title of the dialog is
displayed in a large font at the top
On 2015/07/08 at 15:10:57, Hixie wrote:
> This is an interesting convention (putting the documentation on the fields,
rather than on the constructor). What does the output look like? Is it clear
that these really refer to the constructor arguments?

I was just experimenting with different approaches.  I'm not sure happy with how
any of them look.  I'll show you in person what the output looks like.

> If we document these, we're gonna have to reconsider making the (mutable)
fields public on StatefulComponents.