BUG= https://code.google.com/p/dart/issues/detail?id=21229

Example:
$ cat pubspec.yaml
name: Name
description: A sample command-line application
version: 1.0

$ pub get
Resolving dependencies...
Error on line 3, column 10 of pubspec.yaml: "version" field must have three numeric components: major, minor, and patch. Instead of "1.0", consider "1.0.0".
version: 1.0
         ^^^

also

$ cat pubspec.yaml
name: Name
description: A sample command-line application
version: 1

$ pub get
Resolving dependencies...
Error on line 3, column 10 of pubspec.yaml: "version" field must have three numeric components: major, minor, and patch. Instead of "1", consider "1.0.0".
version: 1
         ^

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//632483002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@40971 260f80e4-7a28-3924-810f-c04153c831b5