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

runtime/vm/snapshot_test.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 Fields  {
   Fields(int i, int j) : fld1 = i, fld2 = j, fld5 = true {}
   int fld1;
   final int fld2;
   static int fld3;
   static final int fld4 = 10;
@@ skipping 103 matching lines @@
   void push(int pile, TowersDisk disk) {
     TowersDisk top = piles[pile];
     if ((top != null) && (disk.size >= top.size))
       Error.error("Cannot put a big disk on a smaller disk.");
     disk.next = top;
     piles[pile] = disk;
   }
 
   TowersDisk pop(int pile) {
     var top = piles[pile];
-    if (top == null)
+    if (top === null)
       Error.error("Attempting to remove a disk from an empty pile.");
     piles[pile] = top.next;
     top.next = null;
     return top;
   }
 
   void moveTop(int from, int to) {
     push(to, pop(from));
     movesDone++;
   }
@@ skipping 382 matching lines @@
 // T r e e S o r t
 class TreeNodePress {
   int value;
   TreeNodePress left;
   TreeNodePress right;
 
   TreeNodePress(int n) : value = n {}
 
   void insert(int n) {
     if (n < value) {
-      if (left == null) left = new TreeNodePress(n);
+      if (left === null) left = new TreeNodePress(n);
       else left.insert(n);
     } else {
-      if (right == null) right = new TreeNodePress(n);
+      if (right === null) right = new TreeNodePress(n);
       else right.insert(n);
     }
   }
 
   void check() {
     TreeNodePress left = this.left;
     TreeNodePress right = this.right;
     int value = this.value;
 
-    return ((left == null) || ((left.value < value) && left.check())) &&
-           ((right == null) || ((right.value >= value) && right.check()));
+    return ((left === null) || ((left.value < value) && left.check())) &&
+           ((right === null) || ((right.value >= value) && right.check()));
   }
 }
 
 class TreeSort {
   static void sort(List<int> a) {
     int len = a.length;
     TreeNodePress tree = new TreeNodePress(a[0]);
     for (var i = 1; i < len; i++) tree.insert(a[i]);
     if (!tree.check()) Error.error("Invalid result, tree not sorted");
   }
@@ skipping 50 matching lines @@
 
   static ListElement makeList(int length) {
     if (length == 0) return null;
     return new ListElement(length, makeList(length - 1));
   }
 
   static bool isShorter(ListElement x, ListElement y) {
     ListElement xTail = x;
     ListElement yTail = y;
     while (yTail != null) {
-      if (xTail == null) return true;
+      if (xTail === null) return true;
       xTail = xTail.next;
       yTail = yTail.next;
     }
     return false;
   }
 }
 
 class Takl {
   static ListElement takl(ListElement x, ListElement y, ListElement z) {
     if (ListElement.isShorter(y, x)) {
@@ skipping 1507 matching lines @@
 }
 
 
 class LineProcessor extends Isolate {
 
   LineProcessor() : super() { }
 
   void main() {
     this.port.receive((message, SendPort replyTo) {
       if (message == MandelIsolateTest.TERMINATION_MESSAGE) {
-        assert(replyTo == null);
+        assert(replyTo === null);
         this.port.close();
       } else {
         replyTo.send(_processLine(message), null);
       }
     });
   }
 
   static List<int> _processLine(int y) {
     double inverseN = 2.0 / MandelIsolateTest.N;
     double Civ = y * inverseN - 1.0;
@@ skipping 38 matching lines @@
       });
     });
   }
 
   void main() {
     this.port.receive((ignored, replyTo) {
       replyTo.send("foo", null);
     });
   }
 }