Don't crash 'pub get' if a dev dependency is broken.

sdk/lib/_internal/pub_generated/lib/src/source/path.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library pub.source.path;
 
 import 'dart:async';
 
 import 'package:path/path.dart' as p;
 
@@ skipping 79 matching lines @@
       return description;
     }
 
     if (description is! String) {
       throw new FormatException("The description must be a path string.");
     }
 
     // Resolve the path relative to the containing file path, and remember
     // whether the original path was relative or absolute.
     var isRelative = p.isRelative(description);
-    if (p.isRelative(description)) {
-      // Can't handle relative paths coming from pubspecs that are not on the
-      // local file system.
-      assert(containingPath != null);
+    if (isRelative) {
+      // Relative paths coming from pubspecs that are not on the local file
+      // system aren't allowed. This can happen if a hosted or git dependency
+      // has a path dependency.
+      if (containingPath == null) {
+        throw new FormatException(
+            '"$description" is a relative path, but this ' 'isn\'t a local pubspec.');
+      }
 
       description = p.normalize(p.join(p.dirname(containingPath), description));
     }
 
     return {
       "path": description,
       "relative": isRelative
     };
   }
 
@@ skipping 35 matching lines @@
     if (fileExists(dir)) {
       fail(
           'Path dependency for package $name must refer to a directory, '
               'not a file. Was "$dir".');
     }
 
     throw new PackageNotFoundException(
         'Could not find package $name at "$dir".');
   }
 }