Create a breadcrumb trail for analyzing of task model extensions.

Create a breadcrumb trail for analyzing of task model extensions.

This change paves the way for modifying
pkg/analyzer/tool/task_dependency_graph.dart so that it will be able to
understand the extensibility mechanisms in the task model.
Specifically, it adds the ExtensionPointId annotation, which
task_dependency_graph.dart will be able to use to locate the
TaskDescriptors that are plugged into each task model extension point.

R=brianwilkerson@google.com

Committed: https://github.com/dart-lang/sdk/commit/3ae9e6228408e462ff863d59edaa74f44185ad48

[Paul Berry, 2015-10-09 16:54:22]

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

[Paul Berry, 2015-10-09 16:54:23]

Once this lands I should be able to fix task_dependency_graph.dart so that it
recognizes task dependencies that occur via extension points.

[Brian Wilkerson, 2015-10-09 17:53:45]

LGTM

[Paul Berry, 2015-10-09 18:05:19]

Committed patchset #1 (id:1) manually as
3ae9e6228408e462ff863d59edaa74f44185ad48 (presubmit successful).