Change how we provide value previews in the vm service.

Change how we provide value previews in the vm service.

Rename the 'preview' property to 'valueAsString'.

Rework printing so that we only add 'valueAsString' when we need it
(num, String, etc.).

Change ToUserCString to be a String-only method.  Drop ToUserCString
for lists -- it was hokey and unused.  Max string length is now 1024
instead of 40.

Update tests.

BUG=
R=johnmccutchan@google.com

Committed: https://code.google.com/p/dart/source/detail?r=34612

[Cutch, 2014-03-31 23:27:14]

lgtm

[turnidge, 2014-04-01 19:46:09]

Committed patchset #3 manually as r34612 (presubmit successful).