Make Expect.throws not accept arguments that fail to be called.

pkg/expect/lib/expect.dart

 // Copyright (c) 2012, 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.
 
 /**
  * This library contains an Expect class with static methods that can be used
  * for simple unit-tests.
  */
 library expect;
 
@@ skipping 328 matching lines @@
    * Calls the function [f] and verifies that it throws an exception.
    * The optional [check] function can provide additional validation
    * that the correct exception is being thrown.  For example, to check
    * the type of the exception you could write this:
    *
    *     Expect.throws(myThrowingFunction, (e) => e is MyException);
    */
   static void throws(void f(),
                      [_CheckExceptionFn check = null,
                       String reason = null]) {
+    String msg = reason == null ? "" : "($reason)";
+    if (f is! _Nullary) {
+      // Only throws from executing the funtion body should count as throwing.
+      // The failure to even call `f` should throw outside the try/catch.
+      _fail("Expect.throws$msg: Funciton f not callable with zero arguments");
+    }
     try {
       f();
     } catch (e, s) {
       if (check != null) {
         if (!check(e)) {
-          String msg = reason == null ? "" : reason;
-          _fail("Expect.throws($msg): Unexpected '$e'\n$s");
+          _fail("Expect.throws$msg: Unexpected '$e'\n$s");
         }
       }
       return;
     }
-    String msg = reason == null ? "" : reason;
-    _fail('Expect.throws($msg) fails');
+    _fail('Expect.throws$msg fails: Did not throw');
   }
 
   static String _getMessage(String reason)
       => (reason == null) ? "" : ", '$reason'";
 
   static void _fail(String message) {
     throw new ExpectException(message);
   }
 }
 
 bool _identical(a, b) => identical(a, b);
 
 typedef bool _CheckExceptionFn(exception);
+typedef _Nullary();  // Expect.throws argument must be this type.
 
 class ExpectException implements Exception {
   ExpectException(this.message);
   String toString() => message;
   String message;
 }