pub: a pubspec file with only comments should not cause errors

[DartBot]

<img src="https://avatars.githubusercontent.com/u/17034?v=3" align="left" width="96" height="96"hspace="10"> Issue by kevmoo
Originally opened as dart-lang/sdk#3483

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What steps will reproduce the problem?

1. Create dart project to import via pub
2. Put in a pubspec with only comments
3. Try 'pub install'
4. Notice the error: 'The pubspec must be a YAML mapping.'

What is the expected output? What do you see instead?
Treat a null value from loadYaml as non-existant pubspec and ignore it

What version of the product are you using? On what operating system?
8370

[DartBot]

<img src="https://avatars.githubusercontent.com/u/17034?v=3" align="left" width="48" height="48"hspace="10"> Comment by kevmoo

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Ready for review: https://chromiumcodereview.appspot.com/10534081

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2108507?v=3" align="left" width="48" height="48"hspace="10"> Comment by dgrove

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Set owner to @munificent.
Added Area-Pub, Triaged labels.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/17034?v=3" align="left" width="48" height="48"hspace="10"> Comment by kevmoo

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FIxed: http://code.google.com/p/dart/source/detail?r=9686

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="48" height="48"hspace="10"> Comment by nex3

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Added Fixed label.