First step for modular output in fasta.

pkg/front_end/lib/src/fasta/kernel/verifier.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.
 
 library fasta.verifier;
 
 import 'package:front_end/src/fasta/type_inference/type_schema.dart'
     show TypeSchemaVisitor, UnknownType;
 
 import 'package:kernel/ast.dart'
@@ skipping 29 matching lines @@
   String fileUri;
 
   FastaVerifyingVisitor(bool isOutline) {
     this.isOutline = isOutline;
   }
 
   @override
   problem(TreeNode node, String details, {TreeNode context}) {
     context ??= this.context;
     VerificationError error = new VerificationError(context, node, details);
-    printUnexpected(Uri.parse(fileUri), node?.fileOffset ?? -1, "$error");
+    var uri = fileUri != null ? Uri.parse(fileUri) : null;
+    var offset = (uri != null && node != null) ? node.fileOffset : -1;
+    printUnexpected(uri, offset, "$error");
     errors.add(error);
   }
 
   @override
   visitExpressionStatement(ExpressionStatement node) {
     // Bypass verification of the [StaticGet] in [RedirectingFactoryBody] as
     // this is a static get without a getter.
     if (node is! RedirectingFactoryBody) {
       super.visitExpressionStatement(node);
     }
@@ skipping 37 matching lines @@
   visitInvalidInitializer(InvalidInitializer node) {
     problem(node, "Invalid initializer.");
   }
 
   @override
   visitUnknownType(UnknownType node) {
     // Note: we can't pass [node] to [problem] because it's not a [TreeNode].
     problem(null, "Unexpected appearance of the unknown type.");
   }
 }