Make expect* throw useful errors outside tests.

lib/src/frontend/expect.dart

 // Copyright (c) 2015, 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.
 
 library test.frontend.expect;
 
 import 'package:matcher/matcher.dart';
 
 import '../backend/closed_exception.dart';
 import '../backend/invoker.dart';
@@ skipping 21 matching lines @@
 /// [matcher] can be a value in which case it will be wrapped in an
 /// [equals] matcher.
 ///
 /// If the assertion fails a [TestFailure] is thrown.
 ///
 /// In some cases extra diagnostic info can be produced on failure (for
 /// example, stack traces on mismatched exceptions). To enable these,
 /// [verbose] should be specified as `true`.
 void expect(actual, matcher,
     {String reason, bool verbose: false, ErrorFormatter formatter}) {
+  if (Invoker.current == null) {
+    throw new StateError("extend() may only be called within a test.");
+  }
+
   if (Invoker.current.closed) throw new ClosedException();
 
   matcher = wrapMatcher(matcher);
   var matchState = {};
   try {
     if (matcher.matches(actual, matchState)) return;
   } catch (e, trace) {
     if (reason == null) {
       reason = '${(e is String) ? e : e.toString()} at $trace';
     }
@@ skipping 15 matching lines @@
 
   var mismatchDescription = new StringDescription();
   matcher.describeMismatch(actual, mismatchDescription, matchState, verbose);
 
   if (mismatchDescription.length > 0) {
     description.add('   Which: ${mismatchDescription}\n');
   }
   if (reason != null) description.add(reason).add('\n');
   return description.toString();
 }