[package:vm] Account for package map comments.

Bug: https://github.com/dart-lang/sdk/issues/36954
Change-Id: Id224e9358cadc1f3c30fe5890515332074ec6983
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/102460
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Alexander Markov <alexmarkov@google.com>
diff --git a/pkg/vm/lib/kernel_front_end.dart b/pkg/vm/lib/kernel_front_end.dart
index aeaea77..6b750c2 100644
--- a/pkg/vm/lib/kernel_front_end.dart
+++ b/pkg/vm/lib/kernel_front_end.dart
@@ -614,6 +614,10 @@
   }
   // file:///a/b/x/y/main.dart -> package:x.y/main.dart
   for (var line in packages) {
+    if (line.isEmpty || line.startsWith("#")) {
+      continue;
+    }
+
     final colon = line.indexOf(':');
     if (colon == -1) {
       continue;