pkg/test: fix error message for expect call outside of test body

lib/src/frontend/expect.dart

Index: lib/src/frontend/expect.dart
diff --git a/lib/src/frontend/expect.dart b/lib/src/frontend/expect.dart
index 8d73ea308b71ede696e0a94fe04d548548fcc120..3059b2faa33b4fa9df96ad93de8b5153bac26566 100644
--- a/lib/src/frontend/expect.dart
+++ b/lib/src/frontend/expect.dart
@@ -40,7 +40,7 @@ typedef String ErrorFormatter(
 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.");
+    throw new StateError("expect() may only be called within a test.");
   }
 
   if (Invoker.current.closed) throw new ClosedException();