Make 'where' lazy.

pkg/unittest/lib/unittest.dart

 // Copyright (c) 2011, 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.
 
 /**
  * A library for writing dart unit tests.
  *
  * To import this library, use the pub package manager.
  * Create a pubspec.yaml file in your project and add
  * a dependency on unittest with the following lines:
@@ skipping 719 matching lines @@
 void filterTests(testFilter) {
   var filterFunction;
   if (testFilter is String) {
     RegExp re = new RegExp(testFilter);
     filterFunction = (t) => re.hasMatch(t.description);
   } else if (testFilter is RegExp) {
     filterFunction = (t) => testFilter.hasMatch(t.description);
   } else if (testFilter is Function) {
     filterFunction = testFilter;
   }
-  _tests = _tests.where(filterFunction);
+  _tests = _tests.where(filterFunction).toList();
 }
 
 /** Runs all queued tests, one at a time. */
 runTests() {
   _currentTest = 0;
   _currentGroup = '';
 
   // If we are soloing a test, remove all the others.
   if (_soloTest != null) {
     filterTests((t) => t == _soloTest);
@@ skipping 149 matching lines @@
 }
 
 /** Enable a test by ID. */
 void enableTest(int testId) => _setTestEnabledState(testId, true);
 
 /** Disable a test by ID. */
 void disableTest(int testId) => _setTestEnabledState(testId, false);
 
 /** Signature for a test function. */
 typedef void TestFunction();