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

runtime/lib/string.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.
 
 /**
  * [StringBase] contains common methods used by concrete String implementations,
  * e.g., OneByteString.
  */
 class StringBase {
 
@@ skipping 117 matching lines @@
     }
     for (int index = fromIndex; index >= 0; index--) {
       if (this.substringMatches(index, other)) {
         return index;
       }
     }
     return -1;
   }
 
   String substring(int startIndex, [int endIndex]) {
-    if (endIndex == null) endIndex = this.length;
+    if (endIndex === null) endIndex = this.length;
 
     if ((startIndex < 0) || (startIndex > this.length)) {
       throw new IndexOutOfRangeException(startIndex);
     }
     if ((endIndex < 0) || (endIndex > this.length)) {
       throw new IndexOutOfRangeException(endIndex);
     }
     if (startIndex > endIndex) {
       throw new IndexOutOfRangeException(startIndex);
     }
@@ skipping 286 matching lines @@
     for (int g in groups) {
       result.add(group(g));
     }
     return result;
   }
 
   final int _start;
   final String str;
   final String pattern;
 }