Fix most strong mode warnings.

CHANGELOG.md

+## 1.9.1
+
+* Fix all strong mode warnings and add generic method annotations.
+
+* `new StreamQueue()` now takes a `Stream<T>` rather than a `Stream<dynamic>`.
+  Passing a type that wasn't `is`-compatible with `Stream<T>` would already
+  throw an error under some circumstances, so this is not considered a breaking
+  change.
+
+* `new SubscriptionStream()` now takes a `Stream<T>` rather than a
+  `Stream<dynamic>`. Passing a type that wasn't `is`-compatible with `Stream<T>`
+  would already throw an error under some circumstances, so this is not
+  considered a breaking change.
+
+* `ErrorResult` now takes a type parameter.
+
+* `Result.asError` now returns a `Result<T>`.
+
 ## 1.9.0
 
 * Deprecate top-level libraries other than `package:async/async.dart`, which
   exports these libraries' interfaces.
 
 * Add `Result.captureStreamTransformer`, `Result.releaseStreamTransformer`,
   `Result.captureSinkTransformer`, and `Result.releaseSinkTransformer`.
 
 * Deprecate `CaptureStreamTransformer`, `ReleaseStreamTransformer`,
   `CaptureSink`, and `ReleaseSink`. `Result.captureStreamTransformer`,
@@ skipping 80 matching lines @@
 - Added a `StreamSplitter` class for splitting a stream into multiple new
   streams.
 
 ## 1.1.1
 
 - Updated SDK version constraint to at least 1.9.0.
 
 ## 1.1.0
 
 - ChangeLog starts here.