Rollback to the older version of crypto to fix build.

pkg/analyzer/lib/src/summary/incremental_cache.dart

Index: pkg/analyzer/lib/src/summary/incremental_cache.dart
diff --git a/pkg/analyzer/lib/src/summary/incremental_cache.dart b/pkg/analyzer/lib/src/summary/incremental_cache.dart
index 34cbd85faa721bed423e91525e8d8bcea93d19de..f9f6c503bf816167d464b007de538b6ad699bcee 100644
--- a/pkg/analyzer/lib/src/summary/incremental_cache.dart
+++ b/pkg/analyzer/lib/src/summary/incremental_cache.dart
@@ -288,6 +288,14 @@ class IncrementalCache {
     byteSink.add(configSalt);
     // Done.
     byteSink.close();
+
+    // TODO(paulberry): this call to `close` should not be needed.
+    // Can be removed once
+    //   https://github.com/dart-lang/crypto/issues/33
+    // is fixed – ensure the min version constraint on crypto is updated, tho.
+    // Does not cause any problems in the mean time.
+    digestSink.close();
+
     return digest.bytes;
   }