Change handling of dart-ext: URIs for native extensions.

Change handling of dart-ext: URIs for native extensions.

With this change, a dart-ext: URI is now an opaque URI consisting of
the scheme dart-ext: followed by an actual URI, either relative, or
with a scheme like file:, http:, or package:.  Existing users are mostly
or all using relative URIs, so this change should not affect them.

This is a step toward allowing native extensions in packages to be served
over http, and also cleans up the code, and would help a future change
that replaces "dart-ext:" with metadata or an annotation.

BUG=
R=iposva@google.com

Committed: https://code.google.com/p/dart/source/detail?r=33216

[Bill Hesse, 2014-02-27 18:50:50]

A refactoring and cleanup of the dart-ext: code.  Needed in order to implement
native extensions in packages with http: URLs, requested by users.

A good cleanup even without it.

[Ivan Posva, 2014-03-03 08:26:37]

LGTM -ip

https://codereview.chromium.org/180783008/diff/1/runtime/bin/builtin.dart
File runtime/bin/builtin.dart (right):

https://codereview.chromium.org/180783008/diff/1/runtime/bin/builtin.dart#new...
runtime/bin/builtin.dart:224: const DART_EXT = 'dart-ext:';
This constant should only be defined once. How about a top-level _DART_EXT?

[Bill Hesse, 2014-03-03 14:51:49]

Committed patchset #2 manually as r33216 (presubmit successful).

[Ivan Posva, 2014-03-03 20:38:00]

-Ivan

https://codereview.chromium.org/180783008/diff/20001/runtime/bin/builtin.dart
File runtime/bin/builtin.dart (right):

https://codereview.chromium.org/180783008/diff/20001/runtime/bin/builtin.dart...
runtime/bin/builtin.dart:208: const _DART_EXT 'dart-ext:';
How does this compile?

[Bill Hesse, 2014-03-04 09:27:13]

https://codereview.chromium.org/180783008/diff/1/runtime/bin/builtin.dart
File runtime/bin/builtin.dart (right):

https://codereview.chromium.org/180783008/diff/1/runtime/bin/builtin.dart#new...
runtime/bin/builtin.dart:224: const DART_EXT = 'dart-ext:';
On 2014/03/03 08:26:37, Ivan Posva wrote:
> This constant should only be defined once. How about a top-level _DART_EXT?

Done.  Committed with a typo in r33216, fixed in r33217.