Use strict equality when comparing with null, especially when null is a default value.

corelib/src/expect.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.
 
 class Expect {
 
   /**
    * Checks whether the expected and actual values are equal (using [:==:]).
    */
   static void equals(var expected, var actual, [String reason = null]) {
@@ skipping 56 matching lines @@
 
   /**
    * Failure if the difference between expected and actual is greater than the
    * given tolerance. If no tolerance is given, tolerance is assumed to be the
    * value 4 significant digits smaller than the value given for expected.
    */
   static void approxEquals(num expected,
                            num actual,
                            [num tolerance = null,
                             String reason = null]) {
-    if (tolerance == null) {
+    if (tolerance === null) {
       tolerance = (expected / Math.pow(10.0, 4.0)).abs();
     }
     // Note: use !( <= ) rather than > so we fail on NaNs
     if ((expected - actual).abs() <= tolerance) return;
 
     String msg = _getMessage(reason);
     _fail('Expect.approxEquals(expected:<$expected>, actual:<$actual>, ' +
           'tolerance:<$tolerance>$msg) fails');
   }
 
@@ skipping 147 matching lines @@
       if (check != null) {
         Expect.isTrue(check(e));
       }
       return;
     }
     String msg = _getMessage(reason);
     _fail('Expect.throws($msg) fails');
   }
 
   static String _getMessage(String reason)
-      => (reason == null) ? "" : ", '$reason'";
+      => (reason === null) ? "" : ", '$reason'";
 
   static void _fail(String message) {
     throw new ExpectException(message);
   }
 }
 
 typedef bool _CheckExceptionFn(exception);
 
 class ExpectException implements Exception {
   ExpectException(this.message);
   String toString() => message;
   String message;
 }