Use Double::NewCanonical instead of creating a double object and then canonicalizing it.

vm/ast.cc

Index: vm/ast.cc
===================================================================
--- vm/ast.cc	(revision 17491)
+++ vm/ast.cc	(working copy)
@@ -93,9 +93,8 @@
       const Double& dbl = Double::Cast(literal());
       // Preserve negative zero.
       double new_value = (dbl.value() == 0.0) ? -0.0 : (0.0 - dbl.value());
-      Double& double_instance =
-          Double::ZoneHandle(Double::New(new_value, Heap::kOld));
-      double_instance ^= double_instance.Canonicalize();
+      const Double& double_instance =
+          Double::ZoneHandle(Double::NewCanonical(new_value));
       return new LiteralNode(this->token_pos(), double_instance);
     }
   }