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

corelib/src/implementation/future_implementation.dart

Index: corelib/src/implementation/future_implementation.dart
===================================================================
--- corelib/src/implementation/future_implementation.dart	(revision 855)
+++ corelib/src/implementation/future_implementation.dart	(working copy)
@@ -75,7 +75,7 @@
   }
 
   bool get hasValue() {
-    return isComplete && exception == null;
+    return isComplete && _exception === null;
   }
 
   void then(void onComplete(T value)) {
@@ -119,7 +119,7 @@
   }
 
   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);
     }