Fix a failing test on Windows.

Pub wasn't canonicalizing a path in the same way as the tests, which
caused a difference on the Windows bots.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1560393002 .

lib/src/pubspec.dart

@@ -348,7 +348,7 @@ class Pubspec {
           // Make the package dir absolute because for the entrypoint it'll just
           // be ".", which may be confusing.
           'Could not find a file named "pubspec.yaml" in '
-              '"${path.normalize(path.absolute(packageDir))}".',
+              '"${canonicalize(packageDir)}".',
           pubspecPath);
     }