Implement hashCode on objects stored in a set or used as map keys.

dart/sdk/lib/_internal/compiler/implementation/util/util.dart

Index: dart/sdk/lib/_internal/compiler/implementation/util/util.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/util/util.dart b/dart/sdk/lib/_internal/compiler/implementation/util/util.dart
index c9c0e6657dd90ffc915bbe9a30d59d1f97ba3f6f..3456dbf4bd337772accee22f3105e05b5224080d 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/util/util.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/util/util.dart
@@ -108,3 +108,10 @@ void writeJsonEscapedCharsOn(String string, buffer) {
   }
   buffer.write(string);
 }
+
+int computeHashCode(part1, [part2, part3, part4]) {
+  return (part1.hashCode
+          ^ part2.hashCode
+          ^ part3.hashCode
+          ^ part4.hashCode) & 0x3fffffff;
+}