Begin to compute constant expressions in resolution.

tests/compiler/dart2js/mock_compiler.dart

Index: tests/compiler/dart2js/mock_compiler.dart
diff --git a/tests/compiler/dart2js/mock_compiler.dart b/tests/compiler/dart2js/mock_compiler.dart
index 3eb6a81f52b592cb75791008ac5bdc1d198208eb..f6c9cc87e433443859e1235213ee62db24650663 100644
--- a/tests/compiler/dart2js/mock_compiler.dart
+++ b/tests/compiler/dart2js/mock_compiler.dart
@@ -9,6 +9,7 @@ import 'dart:async';
 import 'dart:collection';
 
 import 'package:compiler/compiler.dart' as api;
+import 'package:compiler/src/constants/expressions.dart';
 import 'package:compiler/src/elements/elements.dart';
 import 'package:compiler/src/js_backend/js_backend.dart'
     show JavaScriptBackend;
@@ -360,6 +361,12 @@ class CollectingTreeElements extends TreeElementMapping {
   void remove(Node node) {
     map.remove(node);
   }
+
+  List<ConstantExpression> get constants {
+    List<ConstantExpression> list = <ConstantExpression>[];
+    forEachConstantNode((_, c) => list.add(c));
+    return list;
+  }
 }
 
 // The mock compiler does not split the program in output units.