Support overrides in the version solver.

Support overrides in the version solver.

BUG=https://code.google.com/p/dart/issues/detail?id=8566
R=nweiz@google.com

Committed: https://code.google.com/p/dart/source/detail?r=30326

[Bob Nystrom, 2013-11-15 02:15:05]

I'm doing this in small patches so you don't have to review a giant pile of
code. This one just adds support to the solver itself (and tests), but doesn't
expose it to pub. I'll do that in the next patch(es).

[nweiz, 2013-11-15 21:57:16]

lgtm

https://codereview.chromium.org/73463003/diff/1/sdk/lib/_internal/pub/lib/src...
File sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart (right):

https://codereview.chromium.org/73463003/diff/1/sdk/lib/_internal/pub/lib/src...
sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart:101: for (var
override in overrides) {
We should probably detect duplicates in the pubspec parsing code, but it
wouldn't hurt to add "assert(!_overrides.containsKey(override.name))" here.

[Bob Nystrom, 2013-11-16 00:19:09]

https://codereview.chromium.org/73463003/diff/1/sdk/lib/_internal/pub/lib/src...
File sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart (right):

https://codereview.chromium.org/73463003/diff/1/sdk/lib/_internal/pub/lib/src...
sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart:101: for (var
override in overrides) {
On 2013/11/15 21:57:16, nweiz wrote:
> We should probably detect duplicates in the pubspec parsing code, but it
> wouldn't hurt to add "assert(!_overrides.containsKey(override.name))" here.

It's parsed from a map like regular dependencies, so duplicate names shouldn't
be possible. I'm pretty sure other code that works with .dependencies and
.devDependencies just silently assumes they are unique.

[Bob Nystrom, 2013-11-16 00:30:42]

Committed patchset #1 manually as r30326 (presubmit successful).