Fix incorrect strict not-equal comparison when both arguments are constants.

tests/language/strict_equal_test.dart

Index: tests/language/strict_equal_test.dart
===================================================================
--- tests/language/strict_equal_test.dart	(revision 12879)
+++ tests/language/strict_equal_test.dart	(working copy)
@@ -23,6 +23,9 @@
     Expect.equals(2, test5(3));
 
     Expect.equals(1, test6());
+
+    Expect.isFalse(test7());
+    Expect.equals(2, test8());
   }
 }
 
@@ -75,3 +78,16 @@
 get5() {
   return 5;
 }
+
+test7() {
+  return null !== null;
+}
+
+
+test8() {
+  if (null !== null) {
+    return 1;
+  } else {
+    return 2;
+  }
+}