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

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

Index: sdk/lib/_internal/pub_generated/lib/src/source/path.dart
diff --git a/sdk/lib/_internal/pub_generated/lib/src/source/path.dart b/sdk/lib/_internal/pub_generated/lib/src/source/path.dart
index dc1e8028b7b8a6599e2fa3031a233ef218838a8f..d6f7b8c0c70a9ce04e576216835edb1cc805533e 100644
--- a/sdk/lib/_internal/pub_generated/lib/src/source/path.dart
+++ b/sdk/lib/_internal/pub_generated/lib/src/source/path.dart
@@ -97,10 +97,14 @@ class PathSource extends Source {
     // 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));
     }