Constant folding of indexers

pkg/compiler/lib/src/constants/values.dart

Index: pkg/compiler/lib/src/constants/values.dart
diff --git a/pkg/compiler/lib/src/constants/values.dart b/pkg/compiler/lib/src/constants/values.dart
index 2a7c5c4d45e157f3f8896c95aa8fbf6f8cf2aec0..945aaba89e838c506dc52cca41077f467f436237 100644
--- a/pkg/compiler/lib/src/constants/values.dart
+++ b/pkg/compiler/lib/src/constants/values.dart
@@ -502,6 +502,7 @@ class MapConstantValue extends ObjectConstantValue {
   final List<ConstantValue> keys;
   final List<ConstantValue> values;
   final int hashCode;
+  Map<ConstantValue, ConstantValue> _lookupMap;
 
   MapConstantValue(InterfaceType type,
                    List<ConstantValue> keys,
@@ -539,6 +540,12 @@ class MapConstantValue extends ObjectConstantValue {
 
   int get length => keys.length;
 
+  ConstantValue lookup(ConstantValue key) {
+    var lookupMap = _lookupMap ??=
+        new Map<ConstantValue, ConstantValue>.fromIterables(keys, values);
+    return lookupMap[key];
+  }
+
   accept(ConstantValueVisitor visitor, arg) => visitor.visitMap(this, arg);
 
   String unparse() {