Make the license script compatible with recently changed Dart I/O stream APIs (flutter#9725)

Author: jason-simmons

ci/licenses_golden/tool_signature

@@ -1,2 +1,2 @@
-Signature: 4f2f6f3820bc3297f81495a994035ef4
+Signature: f090834bf22f515d09c8d64dc143db47
 

tools/licenses/lib/main.dart

@@ -2258,8 +2258,8 @@ class _Progress {
 Future<String> _readSignature(String goldenPath) async {
   try {
     final system.File goldenFile = system.File(goldenPath);
-    final String goldenSignature = await goldenFile.openRead()
-        .transform(utf8.decoder).transform(const LineSplitter()).first;
+    final String goldenSignature = await utf8.decoder.bind(goldenFile.openRead())
+        .transform(const LineSplitter()).first;
     final RegExp signaturePattern = RegExp(r'Signature: (\w+)');
     final Match goldenMatch = signaturePattern.matchAsPrefix(goldenSignature);
     if (goldenMatch != null)