Begin to compute constant expressions in resolution.

pkg/compiler/lib/src/deferred_load.dart

Index: pkg/compiler/lib/src/deferred_load.dart
diff --git a/pkg/compiler/lib/src/deferred_load.dart b/pkg/compiler/lib/src/deferred_load.dart
index 92116b8f77d792fd51a4a55ad9c908c99ccb6a0d..2f0a82c03d64aeecb78214d97c57ec47f5c0fe3b 100644
--- a/pkg/compiler/lib/src/deferred_load.dart
+++ b/pkg/compiler/lib/src/deferred_load.dart
@@ -286,8 +286,13 @@ class DeferredLoadTask extends CompilerTask {
       }
       treeElements.forEachConstantNode((Node node, _) {
         // Explicitly depend on the backend constants.
-        constants.add(
-            backend.constants.getConstantValueForNode(node, treeElements));
+        ConstantValue value =
+            backend.constants.getConstantValueForNode(node, treeElements);
+        if (value != null) {
+          // TODO(johnniwinther): Assert that all constants have values when
+          // these are directly evaluated.
+          constants.add(value);
+        }
       });
       elements.addAll(treeElements.otherDependencies);
     }