Make pub handle missing git more gracefully.

Make pub handle missing git more gracefully.

- Put in place the basics for reporting errors to the user.
- If you try to install a git package without git installed, you will get
  a nice error.
- Don't run git-related pub tests locally if the user doesn't have git
  installed.

Committed: https://code.google.com/p/dart/source/detail?r=7959

[Bob Nystrom, 2012-05-22 22:44:11]

This re-enables the git tests too. I'll check with Ivan once this goes in and
see if it causes any problems for him. In my local tests where I faked not
having git installed, it seemed to do the right thing.

[nweiz, 2012-05-23 00:46:26]

https://chromiumcodereview.appspot.com/10421026/diff/1/utils/pub/git_source.dart
File utils/pub/git_source.dart (right):

https://chromiumcodereview.appspot.com/10421026/diff/1/utils/pub/git_source.d...
utils/pub/git_source.dart:27: return runProcess("git", ["clone", "--progress",
id.description, destPath],
Line length

https://chromiumcodereview.appspot.com/10421026/diff/1/utils/pub/pub.dart
File utils/pub/pub.dart (right):

https://chromiumcodereview.appspot.com/10421026/diff/1/utils/pub/pub.dart#new...
utils/pub/pub.dart:142: var future = Package.load(workingDir,
cache.sources).chain((package) {
No reason to assign this future to a var; you can just call handleException
directly.

https://chromiumcodereview.appspot.com/10421026/diff/1/utils/pub/pub.dart#new...
utils/pub/pub.dart:153: var message = error.toString();
We should probably have a --trace flag or something similar to force a stack
trace when debugging.

https://chromiumcodereview.appspot.com/10421026/diff/1/utils/pub/pub.dart#new...
utils/pub/pub.dart:169: * command is synchronous, it may return `null`.
What if a synchronous command has an error?

[Bob Nystrom, 2012-05-23 16:32:33]

Thanks!

https://chromiumcodereview.appspot.com/10421026/diff/1/utils/pub/git_source.dart
File utils/pub/git_source.dart (right):

https://chromiumcodereview.appspot.com/10421026/diff/1/utils/pub/git_source.d...
utils/pub/git_source.dart:27: return runProcess("git", ["clone", "--progress",
id.description, destPath],
On 2012/05/23 00:46:26, nweiz wrote:
> Line length

Done.

https://chromiumcodereview.appspot.com/10421026/diff/1/utils/pub/pub.dart
File utils/pub/pub.dart (right):

https://chromiumcodereview.appspot.com/10421026/diff/1/utils/pub/pub.dart#new...
utils/pub/pub.dart:142: var future = Package.load(workingDir,
cache.sources).chain((package) {
On 2012/05/23 00:46:26, nweiz wrote:
> No reason to assign this future to a var; you can just call handleException
> directly.

Done.

https://chromiumcodereview.appspot.com/10421026/diff/1/utils/pub/pub.dart#new...
utils/pub/pub.dart:153: var message = error.toString();
On 2012/05/23 00:46:26, nweiz wrote:
> We should probably have a --trace flag or something similar to force a stack
> trace when debugging.

Added TODO.

https://chromiumcodereview.appspot.com/10421026/diff/1/utils/pub/pub.dart#new...
utils/pub/pub.dart:169: * command is synchronous, it may return `null`.
On 2012/05/23 00:46:26, nweiz wrote:
> What if a synchronous command has an error?

Good call. Added try block around onRun().

[nweiz, 2012-05-23 19:27:20]

lgtm