dart2js cps: Use a global counter to generate hash codes.

pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart

Index: pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart
index 527113a208597136b0ac7419e9ada2155477cfd1..24938d43fc8d9cdd8342d984e711759189f30a43 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart
@@ -23,6 +23,10 @@ abstract class Node {
   /// A pointer to the parent node. Is null until set by optimization passes.
   Node parent;
 
+  /// Workaround for a slow Object.hashCode in the VM.
+  static int _usedHashCodes = 0;
+  final int hashCode = ++_usedHashCodes;
+
   accept(Visitor visitor);
 }