Make expect* throw useful errors outside tests.

CHANGELOG.md

 ## 0.12.3
 
 * If a future matched against the `completes` or `completion()` matcher throws
   an error, that error is printed directly rather than being wrapped in a
   string. This allows such errors to be captured using the Zone API and improves
   formatting.
 
 * In order to be more extensible, all exception handling within tests now uses
   the Zone API.
 
 * Add a heartbeat to reset a test's timeout whenever the test interacts with the
   test infrastructure.
 
+* `expect()`, `expectAsync()`, and `expectAsyncUntil()` throw more useful errors
+  if called outside a test body.
+
 ## 0.12.2
 
 * Convert JavaScript stack traces into Dart stack traces using source maps. This
   can be disabled with the new `--js-trace` flag.
 
 * Improve the browser test suite timeout logic to avoid timeouts when running
   many browser suites at once.
 
 ## 0.12.1
 
@@ skipping 145 matching lines @@
 * **NEW!** `expectAsyncUntil` replaces the now deprecated `expectAsyncUntil0`,
     `expectAsyncUntil1` and `expectAsyncUntil2`
 * `TestCase`:
     * Removed properties: `setUp`, `tearDown`, `testFunction`
     * `enabled` is now get-only
     * Removed methods: `pass`, `fail`, `error`
 * `interactive_html_config.dart` has been removed.
 * `runTests`, `tearDown`, `setUp`, `test`, `group`, `solo_test`, and
   `solo_group` now throw a `StateError` if called while tests are running.
 * `rerunTests` has been removed.