Add test for expected source mappings

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

 // Copyright (c) 2015, 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.
 
 /// Source information system mapping that attempts a semantic mapping between
 /// offsets of JavaScript code points to offsets of Dart code points.
 
 library dart2js.source_information.position;
 
 import '../common.dart';
@@ skipping 524 matching lines @@
       SourceLocation sourceLocation =
           getSourceLocation(sourceInformation, sourcePositionKind);
       if (sourceLocation != null) {
         sourceMapper.register(node, codeLocation, sourceLocation);
       }
     }
 
     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:
         registerPosition(SourcePositionKind.INNER);
         break;
       case StepKind.CALL:
         CallPosition callPosition =
             CallPosition.getSemanticPositionForCall(node);
         registerPosition(callPosition.sourcePositionKind);
         break;
@@ skipping 749 matching lines @@
 
   @override
   CodePosition operator [](js.Node node) {
     CodePosition codePosition = codePositions[node];
     if (codePosition == null) {
       coverage.registerNodesWithoutOffset(node);
     }
     return codePosition;
   }
 }