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 94a0ce07d0e4cd95b5c0f9388071fde3abf68c7f..c9c8399134a5918df15bfa47901ab71f1917b0b7 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/util/util.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/util/util.dart
@@ -110,3 +110,10 @@ void writeJsonEscapedCharsOn(String string, buffer) {
   }
   buffer.write(string);
 }
+
+int computeHashCode(part1, [part2, part3, part4]) {
+  return (part1.hashCode
+          ^ part2.hashCode
+          ^ part3.hashCode
+          ^ part4.hashCode) & 0x3fffffff;
+}