Don't upgrade source_span to avoid formatting changes

Author: nex3

pubspec.yaml

@@ -21,7 +21,9 @@ dependencies:
   package_resolver: "^1.0.0"
   path: "^1.6.0"
   source_maps: "^0.10.5"
-  source_span: "^1.4.0"
+
+  # Temporarily limit this until #926 lands.
+  source_span: ">=1.4.0 <1.6.0"
   stack_trace: ">=0.9.0 <2.0.0"
   stream_transform: ">=0.0.20 <2.0.0"
   string_scanner: ">=0.1.5 <2.0.0"