Be more explicit about JSON error handling.

test/json_document_transformer_test.dart

Index: test/json_document_transformer_test.dart
diff --git a/test/json_document_transformer_test.dart b/test/json_document_transformer_test.dart
index fec3d2ac115aba52f3f452edaed1dbfe24bfab4f..65a3497245cbed111a38039fee00d235027e82c8 100644
--- a/test/json_document_transformer_test.dart
+++ b/test/json_document_transformer_test.dart
@@ -47,4 +47,16 @@ void main() {
     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>()));
+  });
 }