Use Expect.throws for directory test

tests/standalone/src/DirectoryTest.dart

Index: tests/standalone/src/DirectoryTest.dart
diff --git a/tests/standalone/src/DirectoryTest.dart b/tests/standalone/src/DirectoryTest.dart
index 9ebe922064d3180a1f568c2a3130c8ea3e0c9f3a..d015ea6868654c8dd89a8c90d3991d8b1ae1826a 100644
--- a/tests/standalone/src/DirectoryTest.dart
+++ b/tests/standalone/src/DirectoryTest.dart
@@ -264,13 +264,8 @@ String illegalTempDirectoryLocation() {
 testCreateTempErrorSync() {
   var location = illegalTempDirectoryLocation();
   if (location != null) {
-    bool exception = false;
-    try {
-      new Directory(location).createTempSync();
-    } catch(DirectoryException e) {
-      exception = true;
-    }
-    Expect.isTrue(exception);
+    Expect.throws(new Directory(location).createTempSync,
+                  (e) => e is DirectoryException);
   }
 }