Show a better error message on no version errors.

Show a better error message on no version errors.

(Also includes a little solver optimization I stumbled onto.)

BUG=https://code.google.com/p/dart/issues/detail?id=18300
R=nweiz@google.com

Committed: https://code.google.com/p/dart/source/detail?r=35230

[nweiz, 2014-04-21 21:38:49]

lgtm

https://codereview.chromium.org/242373006/diff/1/sdk/lib/_internal/pub/lib/sr...
File sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart (right):

https://codereview.chromium.org/242373006/diff/1/sdk/lib/_internal/pub/lib/sr...
sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart:147:
cache.writeResults(buffer);
I really hate passing around string buffers. Just return a string; the
efficiency difference is effectively nil, and it makes function boundaries much
cleaner.

https://codereview.chromium.org/242373006/diff/1/sdk/lib/_internal/pub/lib/sr...
File sdk/lib/_internal/pub/lib/src/solver/version_solver.dart (right):

https://codereview.chromium.org/242373006/diff/1/sdk/lib/_internal/pub/lib/sr...
sdk/lib/_internal/pub/lib/src/solver/version_solver.dart:209: //
_debugWritePackageGraph(buffer);
Rather than commenting this out, maybe hide it behind a hidden global option or
an environment variable?

[Bob Nystrom, 2014-04-21 22:57:00]

Committed patchset #1 manually as r35230 (presubmit successful).

[Bob Nystrom, 2014-04-21 22:58:32]

https://codereview.chromium.org/242373006/diff/1/sdk/lib/_internal/pub/lib/sr...
File sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart (right):

https://codereview.chromium.org/242373006/diff/1/sdk/lib/_internal/pub/lib/sr...
sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart:147:
cache.writeResults(buffer);
On 2014/04/21 21:38:49, nweiz wrote:
> I really hate passing around string buffers. Just return a string; the
> efficiency difference is effectively nil, and it makes function boundaries
much
> cleaner.

I actually like passing around the buffer because it makes newline handling a
little clearer to me, but done.

https://codereview.chromium.org/242373006/diff/1/sdk/lib/_internal/pub/lib/sr...
File sdk/lib/_internal/pub/lib/src/solver/version_solver.dart (right):

https://codereview.chromium.org/242373006/diff/1/sdk/lib/_internal/pub/lib/sr...
sdk/lib/_internal/pub/lib/src/solver/version_solver.dart:209: //
_debugWritePackageGraph(buffer);
On 2014/04/21 21:38:49, nweiz wrote:
> Rather than commenting this out, maybe hide it behind a hidden global option
or
> an environment variable?

It's not quite ready for prime-time. I slapped it together to help me track down
this bug, but didn't put any time into beyond that. I thought about just
deleting it entirely, but I figured it may be helpful later. I didn't want to
put more time into it now, though, so this was the compromise I came up with.