First step for modular output in fasta.

pkg/front_end/lib/src/fasta/kernel/verifier.dart

Index: pkg/front_end/lib/src/fasta/kernel/verifier.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/verifier.dart b/pkg/front_end/lib/src/fasta/kernel/verifier.dart
index 91b691a8925c7732cdb76844cdb216afcbfa0de5..691b46351967123ad5bc2698c397b64464b8f539 100644
--- a/pkg/front_end/lib/src/fasta/kernel/verifier.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/verifier.dart
@@ -47,7 +47,9 @@ class FastaVerifyingVisitor extends VerifyingVisitor
   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);
   }