dart2js-kernel: do not crash on missing const initializers

pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart

 // Copyright (c) 2016, 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.
 
 import 'package:js_runtime/shared/embedded_names.dart';
 import 'package:kernel/ast.dart' as ir;
 
 import 'dart:collection' show Queue;
 
 import '../common.dart';
@@ skipping 302 matching lines @@
     return TypeMaskFactory.inferredTypeForSelector(
         selector, mask, _globalInferenceResults);
   }
 
   TypeMask typeFromNativeBehavior(
       native.NativeBehavior nativeBehavior, ClosedWorld closedWorld) {
     return TypeMaskFactory.fromNativeBehavior(nativeBehavior, closedWorld);
   }
 
   ConstantValue getConstantFor(ir.Node node) {
+    // Some `null`s are not mapped when they correspond to errors, e.g. missing
+    // `const` initializers.
+    if (node is ir.NullLiteral) return new NullConstantValue();
+
     ConstantValue constantValue =
         _backend.constants.getConstantValueForNode(getNode(node), elements);
     assert(invariant(getNode(node), constantValue != null,
         message: 'No constant computed for $node'));
     return constantValue;
   }
 
   ConstantValue getConstantForParameterDefaultValue(ir.Node defaultExpression) {
     // TODO(27394): Evaluate constant expressions in ir.Node domain.
     // In the interim, expand the Constantifier and do this:
@@ skipping 630 matching lines @@
 
   @override
   String get name => null;
 
   @override
   int get nestingLevel => 1;
 
   @override
   ast.Node get statement => null;
 }