Skip spurious constants in deferred computation.

pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart

Index: pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
diff --git a/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart b/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
index 1baa388c4f9d1c473a6b7f7920ae65a537b9658e..1cac7aabaf32c4880406aa132b9a7b6d47371b2b 100644
--- a/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
+++ b/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
@@ -853,13 +853,12 @@ class TypeGraphInferrerEngine
             // For non-container types, the constant handler does
             // constant folding that could give more precise results.
             ConstantExpression constant = fieldElement.constant;
-            // TODO(johnniwinther): ensure `value` is not null. Note that
-            // calling `evaluate` in getConstantValue may trigger issues for
-            // deferred loading (dartbug.com/26406).
-            ConstantValue value = constant == null
-                ? null
-                : compiler.backend.constants.getConstantValue(constant);
-            if (value != null) {
+            if (constant != null) {
+              ConstantValue value =
+                  compiler.backend.constants.getConstantValue(constant);
+              assert(invariant(fieldElement, value != null,
+                  message: "Constant expression without value: "
+                           "${constant.toStructuredText()}."));
               if (value.isFunction) {
                 FunctionConstantValue functionConstant = value;
                 type = types.allocateClosure(node, functionConstant.element);