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

corelib/src/implementation/future_implementation.dart

 // Copyright 2011 Google Inc. All Rights Reserved.
 // Dart core library.
 
 /**
  * Thrown if client tries to obtain value or exception
  * before a future has completed.
  */
 class FutureNotCompleteException implements Exception {
   FutureNotCompleteException() {}
 }
@@ skipping 57 matching lines @@
       throw new FutureNotCompleteException();
     }
     return _exception;
   }
 
   bool get isComplete() {
     return _isComplete;
   }
 
   bool get hasValue() {
-    return isComplete && exception == null;
+    return isComplete && _exception === null;
   }
 
   void then(void onComplete(T value)) {
     if (hasValue) {
       onComplete(value);
     } else {
       _listeners.add(onComplete);
     }
   }
 
@@ skipping 23 matching lines @@
 
   void _setValue(T value) {
     if (_isComplete) {
       throw new FutureAlreadyCompleteException();
     }
     _value = value;
     _complete();
   }
 
   void _setException(var exception) {
-    if (exception == null) {
+    if (exception === null) {
       // null is not a legal value for the exception of a Future
       throw new IllegalArgumentException(null);
     }
     if (_isComplete) {
       throw new FutureAlreadyCompleteException();
     }
     _exception = exception;
     _complete();
   }
 }
 
 class CompleterImpl<T> implements Completer<T> {
 
   final FutureImpl<T> _futureImpl;
 
   CompleterImpl() : _futureImpl = new FutureImpl() {}
 
   Future<T> get future() {
     return _futureImpl;
   }
 
   void complete(T value) {
     _futureImpl._setValue(value);
   }
 
   void completeException(var exception) {
     _futureImpl._setException(exception);
   }
 }