Rework task model graphing to work on buildbots.

Rework task model graphing to work on buildbots.

This CL reworks the task model grapher (formerly at
pkg/analyzer/tool/task_dependency_graph.dart, now at
pkg/analyzer/tool/task_dependency_graph/generate.dart) so that it
works similarly to the code generation in the analysis server.
Namely, the output file is stored in source control
(pkg/analyzer/tool/task_dependency_graph/tasks.dot), and a test run by
the buildbots verifies that it is up to date.  If it is not, the test
outputs the command necessary to update it.

This carries the benefit that:

- Anyone with access to an installation of graphviz can generate the
  task model graph easily from its .dot file.

- Any time a change occurs that affects task model dependencies, a
  test failure will prompt us that we need to generate a new .dot
  file.

- If a change ever occurs that breaks the ability of the task
  dependency grapher to produce the graph, a test failure will prompt
  us.

The downside is that when making a change that affects task model
dependencies, it is now necessary to re-run the script
pkg/analyzer/tool/task_dependency_graph/generate.dart to avoid a test
failure.

R=brianwilkerson@google.com, scheglov@google.com

Committed: https://github.com/dart-lang/sdk/commit/c5f4af4df6c252f55fb2c08897e5d795c4ee92a7

[Paul Berry, 2015-10-16 16:23:03]

paulberry@google.com changed reviewers:
+ brianwilkerson@google.com, scheglov@google.com

[Brian Wilkerson, 2015-10-16 16:32:30]

> The downside is that when making a change that affects task model
> dependencies, it is now necessary to re-run the script
> pkg/analyzer/tool/task_dependency_graph/generate.dart to avoid a
> test failure.

The bigger downside is that if we need to make a change to the task model that
breaks its ability to produce a graph we have to fix the graph builder as part
of making that change. That probably won't happen often, so I'm willing to wait
and see.

LGTM

[scheglov, 2015-10-16 17:26:24]

LGTM

Having a shell script would be perfect :-)

[Paul Berry, 2015-10-19 16:24:08]

On 2015/10/16 17:26:24, scheglov wrote:
> LGTM
> 
> Having a shell script would be perfect :-)

Done.

[Paul Berry, 2015-10-19 16:24:23]

Committed patchset #1 (id:1) manually as
c5f4af4df6c252f55fb2c08897e5d795c4ee92a7 (presubmit successful).