Add an IO implementation of WebSocketChannel.

lib/src/exception.dart

Index: lib/src/exception.dart
diff --git a/lib/src/exception.dart b/lib/src/exception.dart
index d06dc60e9db21eda7179cdcfeb77ff991b450890..47545ede721cd24b78f1d29f0027edee376c2782 100644
--- a/lib/src/exception.dart
+++ b/lib/src/exception.dart
@@ -11,7 +11,11 @@ class WebSocketChannelException implements Exception {
   /// The exception that caused this one, if available.
   final inner;
 
-  WebSocketChannelException([this.message, this.inner]);
+  WebSocketChannelException([this.message]) : inner = null;
+
+  WebSocketChannelException.from(inner)
+      : message = inner.toString(),
+        inner = inner;
 
   String toString() => message == null
       ? "WebSocketChannelException" :