Put an upper bound on flx dependency

In case we run into trouble, this at least gives us an escape hatch.

packages/flutter_tools/pubspec.yaml

@@ -12,7 +12,7 @@ dependencies:
   args: ^0.13.0
   asn1lib: ^0.4.1
   cipher: ^0.7.1
-  flx: ">=0.0.1"
+  flx: ">=0.0.1 <0.1.0"
   crypto: ^0.9.1
   mustache4dart: ^1.0.0
   path: ^1.3.0