[core] cleanup === and !==

lib/coreimpl/stopwatch_implementation.dart

Index: lib/coreimpl/stopwatch_implementation.dart
===================================================================
--- lib/coreimpl/stopwatch_implementation.dart	(revision 13856)
+++ lib/coreimpl/stopwatch_implementation.dart	(working copy)
@@ -17,11 +17,11 @@
   StopwatchImplementation() : _start = null, _stop = null {}
 
   void start() {
-    if (_start === null) {
+    if (_start == null) {
       // This stopwatch has never been started.
       _start = _now();
     } else {
-      if (_stop === null) {
+      if (_stop == null) {
         return;
       }
       // Restarting this stopwatch. Prepend the elapsed time to the current
@@ -32,18 +32,18 @@
   }
 
   void stop() {
-    if (_start === null || _stop !== null) {
+    if (_start == null || _stop != null) {
       return;
     }
     _stop = _now();
   }
 
   void reset() {
-    if (_start === null) return;
+    if (_start == null) return;
     // If [_start] is not null, then the stopwatch had already been started. It
     // may running right now.
     _start = _now();
-    if (_stop !== null) {
+    if (_stop != null) {
       // The watch is not running. So simply set the [_stop] to [_start] thus
       // having an elapsed time of 0.
       _stop = _start;
@@ -51,10 +51,10 @@
   }
 
   int elapsed() {
-    if (_start === null) {
+    if (_start == null) {
       return 0;
     }
-    return (_stop === null) ? (_now() - _start) : (_stop - _start);
+    return (_stop == null) ? (_now() - _start) : (_stop - _start);
   }
 
   int elapsedInUs() {