Use JavaScript runtime semantics when constant folding.

tests/corelib/expression_test.dart

Index: tests/corelib/expression_test.dart
diff --git a/tests/corelib/expression_test.dart b/tests/corelib/expression_test.dart
index 61d8ccc4b7f0dca38b776f7cacd7c00853c3bc45..910a0c2d7f5d5e05a1769cc7a5ef20d7a550f879 100644
--- a/tests/corelib/expression_test.dart
+++ b/tests/corelib/expression_test.dart
@@ -30,10 +30,10 @@ class ExpressionTest {
   }
 
   testUnary() {
-    int x = 4, y = 2;
+    int x = 4, y = 2, z = -5;
     bool t = true, f = false;
     Expect.equals(-4, -x);
-    Expect.equals(-5, ~x);
+    Expect.equals(4, ~z);
     Expect.equals(f, !t);
   }