Be more explicit about JSON error handling.

test/json_document_transformer_test.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 import 'dart:convert';
 
 import 'package:stream_channel/stream_channel.dart';
 import 'package:test/test.dart';
 
@@ skipping 29 matching lines @@
     streamController.add('{"foo": "bar"}');
     expect(transformed.stream.first, completion(equals("decoded")));
   });
 
   test("supports the toEncodable function", () {
     var transformed = channel.transform(
         new JsonDocumentTransformer(toEncodable: (object) => "encoded"));
     transformed.sink.add(new Object());
     expect(sinkController.stream.first, completion(equals('"encoded"')));
   });
+
+  test("emits a stream error when incoming JSON is malformed", () {
+    var transformed = channel.transform(jsonDocument);
+    streamController.add("{invalid");
+    expect(transformed.stream.first, throwsFormatException);
+  });
+
+  test("synchronously throws if an unencodable object is added", () {
+    var transformed = channel.transform(jsonDocument);
+    expect(() => transformed.sink.add(new Object()),
+        throwsA(new isInstanceOf<JsonUnsupportedObjectError>()));
+  });
 }