Rename internal Dart content handler unit testing library to _testing

Rename internal Dart content handler unit testing library to _testing

- Also, fix clean builds, by not including the sdk_ext_directory as an input dependency because there is no rule to create it.

Committed: https://chromium.googlesource.com/external/mojo/+/d610ed3d266c2a06af0dc855b92373c2a5447181

[Cutch, 2015-05-21 16:24:28]

johnmccutchan@google.com changed reviewers:
+ eseidel@google.com

[Cutch, 2015-05-21 16:55:30]

TBR

[Cutch, 2015-05-21 16:56:03]

Committed patchset #2 (id:20001) manually as
d610ed3d266c2a06af0dc855b92373c2a5447181.

[APW, 2015-05-21 16:57:57]

On 2015/05/21 16:56:03, Cutch wrote:
> Committed patchset #2 (id:20001) manually as
> d610ed3d266c2a06af0dc855b92373c2a5447181.

lgtm