pkg/unittest: Moved `fail` to matcher library

pkg/unittest/lib/src/core_matchers.dart

Index: pkg/unittest/lib/src/core_matchers.dart
diff --git a/pkg/unittest/lib/src/core_matchers.dart b/pkg/unittest/lib/src/core_matchers.dart
index c412450012140c2a662649a142b4650c4a5feec4..236da4e5fb69dcb9ce987870b44b6af0e15433d1 100644
--- a/pkg/unittest/lib/src/core_matchers.dart
+++ b/pkg/unittest/lib/src/core_matchers.dart
@@ -325,8 +325,7 @@ class Throws extends BaseMatcher {
       // Queue up an asynchronous expectation that validates when the future
       // completes.
       item.then((value) {
-        done(() => expect(false, isTrue, reason:
-            "Expected future to fail, but succeeded with '$value'."));
+        done(() => fail("Expected future to fail, but succeeded with '$value'."));
       }, onError: (e) {
         done(() {
           if (_matcher == null) return;