pkg/logging: use UnmodifiableMapView from dart:collection

pkg/logging: use UnmodifiableMapView from dart:collection

remove documentation URL
remove (now) unused collection pkg import
move intro docs to README.md

R=sethladd@google.com

Committed: https://code.google.com/p/dart/source/detail?r=38546

[sethladd, 2014-07-24 16:48:13]

lgtm with some suggestions

https://codereview.chromium.org/413193002/diff/1/pkg/logging/README.md
File pkg/logging/README.md (right):

https://codereview.chromium.org/413193002/diff/1/pkg/logging/README.md#newcode1
pkg/logging/README.md:1: Support for logging.
Nit: start line with # ?

https://codereview.chromium.org/413193002/diff/1/pkg/logging/README.md#newcode2
pkg/logging/README.md:2: 
READMEs often include notes about where to file bugs and the LICENSE

[kevmoo, 2014-07-24 17:44:18]

https://codereview.chromium.org/413193002/diff/1/pkg/logging/README.md
File pkg/logging/README.md (right):

https://codereview.chromium.org/413193002/diff/1/pkg/logging/README.md#newcode1
pkg/logging/README.md:1: Support for logging.
On 2014/07/24 16:48:13, sethladd wrote:
> Nit: start line with # ?

Done.

https://codereview.chromium.org/413193002/diff/1/pkg/logging/README.md#newcode2
pkg/logging/README.md:2: 
On 2014/07/24 16:48:13, sethladd wrote:
> READMEs often include notes about where to file bugs and the LICENSE

There's already a license file in the package. We don't have "contributing" docs
in most other readmes (including where to file bugs). Perhaps we should go the
way of github and have the convention of using CONTRIBUTING.md in a package to
include details about filing issues and patches

[kevmoo, 2014-07-24 17:44:34]

Committed patchset #2 manually as r38546 (presubmit successful).