Add some more discussion of asynchrony.

json_reporter.md

-JSON Reporter Protocol
+xxJSON Reporter Protocol

[kevmoo, 2015/11/24 05:05:11]

oops?

 ======================
 
 The test runner supports a JSON reporter which provides a machine-readable
 representation of the test runner's progress. This reporter is intended for use
 by IDEs and other tools to present a custom view of the test runner's operation
 without needing to parse output intended for humans.
 
+Note that the test runner is highly asynchronous, and users of this protocol
+shouldn't make assumptions about the ordering of events beyond what's explicitly
+specified in this document. It's possible for events from multiple tests to be
+intertwined, for a single test to emit an error after it completed successfully,
+and so on.
+
 ## Usage
 
 Pass the `--reporter json` command-line flag to the test runner to activate the
 JSON reporter.
 
     pub run test --reporter json <path-to-test-file>
 
 The JSON stream will be emitted via standard output. It will be a stream of JSON
 objects, separated by newlines.
 
@@ skipping 218 matching lines @@
 class Metadata {
   // Whether the test case will be skipped by the test runner.
   bool skip;
 
   // The reason the test case is skipped, if the user provided it.
   String? skipReason;
 }
 ```
 
 The metadata attached to a test by a user.