Do not evaluate on-demand to prevent breaking the deferred loading task

pkg/compiler/lib/src/deferred_load.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library deferred_load;
 
 import 'common/backend_api.dart' show Backend;
 import 'common/tasks.dart' show CompilerTask;
 import 'common.dart';
 import 'compiler.dart' show Compiler;
@@ skipping 321 matching lines @@
         if (analyzableElement.resolvedAst.kind != ResolvedAstKind.PARSED) {
           return;
         }
 
         TreeElements treeElements = analyzableElement.resolvedAst.elements;
         assert(treeElements != null);
 
         treeElements
             .forEachConstantNode((Node node, ConstantExpression expression) {
           // Explicitly depend on the backend constants.
-          ConstantValue value = backend.constants.getConstantValue(expression);
-          assert(invariant(node, value != null,
-              message:
-                  "No constant value for ${expression.toStructuredText()}."));
-          constants.add(value);
+          ConstantValue value;
+          value = backend.constants.getConstantValue(expression);
+          // TODO(johnniwinther): ensure `value` is not null or use directly the
+          // expression to calculate dependencies. See dartbug.com/26406 for
+          // context.
+          if (value != null) {
+            constants.add(value);
+          }
         });
       }
     }
 
     // TODO(sigurdm): How is metadata on a patch-class handled?
     for (MetadataAnnotation metadata in element.metadata) {
       ConstantValue constant =
           backend.constants.getConstantValueForMetadata(metadata);
       if (constant != null) {
         constants.add(constant);
@@ skipping 626 matching lines @@
     return result;
   }
 
   bool operator ==(other) {
     if (other is! _DeclaredDeferredImport) return false;
     return declaration == other.declaration;
   }
 
   int get hashCode => declaration.hashCode * 17;
 }