dart2js-kernel: do not crash on missing const initializers

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

Index: pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
diff --git a/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart b/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
index 326359df4e7dfb05f8d745e84ebc2ebd6b5055c4..2d8106f54760ce3ac94bca5fc3e52462492e0fd7 100644
--- a/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
+++ b/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
@@ -320,6 +320,10 @@ class KernelAstAdapter {
   }
 
   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,