Improve --print-ast printing of LetNode.

Improve --print-ast printing of LetNode.

I was trying to debug an issue noticed that printing of LetNode is
kind of useless.  It didn't print the variables though they had
references, which seems confusing, and it writes all the initializers
and body nodes at the same nesting level which makes it impossible to
see where one ends and the other begins.

BUG=
R=vegorov@google.com

Committed: https://github.com/dart-lang/sdk/commit/9d495064f7d5e6a25b1d221bde6858831ad98cbc

[Kevin Millikin (Google), 2017-06-20 14:43:57]

kmillikin@google.com changed reviewers:
+ vegorov@google.com

[Vyacheslav Egorov (Google), 2017-06-20 14:47:23]

lgtm

[Kevin Millikin (Google), 2017-06-20 14:49:17]

Description was changed from

==========
Improve --print-ast printing of LetNode.

I was trying to debug an issue noticed that printing of LetNode is
kind of useless.  It didn't print the variables though they had
references, which seems confusing, and it writes all the initializers
and body nodes at the same nesting level which makes it impossible to
see where one ends and the other begins.

BUG=
==========

to

==========
Improve --print-ast printing of LetNode.

I was trying to debug an issue noticed that printing of LetNode is
kind of useless.  It didn't print the variables though they had
references, which seems confusing, and it writes all the initializers
and body nodes at the same nesting level which makes it impossible to
see where one ends and the other begins.

BUG=
R=vegorov@google.com

Committed:
https://github.com/dart-lang/sdk/commit/9d495064f7d5e6a25b1d221bde6858831ad98cbc
==========

[Kevin Millikin (Google), 2017-06-20 14:49:18]

Committed patchset #1 (id:1) manually as
9d495064f7d5e6a25b1d221bde6858831ad98cbc (presubmit successful).