Support string_scanner 1.0.0.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//2086743002 .

pubspec.yaml

 name: test
-version: 0.12.14-dev
+version: 0.12.14
 author: Dart Team <misc@dartlang.org>
 description: A library for writing dart unit tests.
 homepage: https://github.com/dart-lang/test
@@ -25,7 +25,7 @@ dependencies:
   source_span: '^1.0.0'
   stack_trace: '^1.2.1'
   stream_channel: '^1.3.1'
-  string_scanner: '^0.1.1'
+  string_scanner: '>=0.1.1 <2.0.0'
   web_socket_channel: '^1.0.0'
   yaml: '>=0.9.0 <3.0.0'