add null-aware operators to the language tour

add null-aware operators to the language tour

Staged at:
https://ch02-null-dot-dart-lang.appspot.com/docs/dart-up-and-running/preface.html#changes-after-the-third-printing
https://ch02-null-dot-dart-lang.appspot.com/docs/dart-up-and-running/ch02.html

BUG= https://github.com/dart-lang/www.dartlang.org/issues/1364
R=gbracha@google.com

Committed: c56e3641d2657c5da260eb717bdd095e75342eb1

[Kathy Walrath, 2015-08-20 22:20:29]

kathyw@google.com changed reviewers:
+ gbracha@google.com, sethladd@google.com

[Kathy Walrath, 2015-08-20 22:22:52]

Gilad and/or Seth, could you please take a look?

[gbracha, 2015-08-20 23:44:16]

One minor comment; otherwise lgtm

https://chromiumcodereview.appspot.com/1301343002/diff/1/src/site/docs/dart-u...
File src/site/docs/dart-up-and-running/ch02.markdown (right):

https://chromiumcodereview.appspot.com/1301343002/diff/1/src/site/docs/dart-u...
src/site/docs/dart-up-and-running/ch02.markdown:1273: To assign only if the
assigned-to object is null,
Objects aren;t assigned to - variables are. So this should read "if the
assigned-to variable is null" etc.

[Kathy Walrath, 2015-08-20 23:56:52]

Thanks, Gilad! I have a question...

https://chromiumcodereview.appspot.com/1301343002/diff/1/src/site/docs/dart-u...
File src/site/docs/dart-up-and-running/ch02.markdown (right):

https://chromiumcodereview.appspot.com/1301343002/diff/1/src/site/docs/dart-u...
src/site/docs/dart-up-and-running/ch02.markdown:1273: To assign only if the
assigned-to object is null,
On 2015/08/20 23:44:16, gbracha wrote:
> Objects aren;t assigned to - variables are. So this should read "if the
> assigned-to variable is null" etc.

What if you have an expression such as:

a?.b?.c = 'something';

a?.b doesn't seem like a variable to me. (On the other hand, I'm not showing
that, so "variable" seems fine in this context. I just wonder what would be the
correct way of saying it if I were showing that.)

[Kathy Walrath, 2015-08-21 00:31:08]

https://chromiumcodereview.appspot.com/1301343002/diff/1/src/site/docs/dart-u...
File src/site/docs/dart-up-and-running/ch02.markdown (right):

https://chromiumcodereview.appspot.com/1301343002/diff/1/src/site/docs/dart-u...
src/site/docs/dart-up-and-running/ch02.markdown:1273: To assign only if the
assigned-to object is null,
On 2015/08/20 23:44:16, gbracha wrote:
> Objects aren;t assigned to - variables are. So this should read "if the
> assigned-to variable is null" etc.

Done.

[Kathy Walrath, 2015-08-21 00:32:28]

Committed patchset #2 (id:20001) manually as
c56e3641d2657c5da260eb717bdd095e75342eb1 (presubmit successful).