Add test for expected source mappings

pkg/compiler/lib/src/io/position_information.dart

Index: pkg/compiler/lib/src/io/position_information.dart
diff --git a/pkg/compiler/lib/src/io/position_information.dart b/pkg/compiler/lib/src/io/position_information.dart
index 17e83978dd51fbd065e305e29977e4b029724188..2f67aa35e89bff1b8a122e1407a4423806d0119e 100644
--- a/pkg/compiler/lib/src/io/position_information.dart
+++ b/pkg/compiler/lib/src/io/position_information.dart
@@ -542,7 +542,9 @@ class PositionTraceListener extends TraceListener
     switch (kind) {
       case StepKind.FUN_ENTRY:
         // TODO(johnniwinther): Remove this when fully transitioned to the
-        // new source info system.
+        // new source info system. Verify that tools no longer expect JS
+        // function signatures to map to the origin. The main method may still
+        // need mapping to enable breakpoints before calling main.
         registerPosition(SourcePositionKind.START);
         break;
       case StepKind.FUN_EXIT: