Modify summary Builder classes to avoid the use of Object.

Modify summary Builder classes to avoid the use of Object.

Instead of passing around opaque Objects which are the result of
calling the Builder.finish() methods, we pass around the Builder
objects themselves, and we call finish() from within the generated
code.  This pushes the lack of type safety into the generated code,
leaving the handwritten code type safe.

R=brianwilkerson@google.com

Committed: https://github.com/dart-lang/sdk/commit/178cebb634b0aff0ebae76a1f97247361c1ee92e

[Paul Berry, 2015-11-17 22:37:27]

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

[Brian Wilkerson, 2015-11-17 23:44:27]

LGTM

[Paul Berry, 2015-11-17 23:50:30]

Committed patchset #1 (id:1) manually as
178cebb634b0aff0ebae76a1f97247361c1ee92e (presubmit successful).