Better DDC sourcemap generation for lambdas

pkg/dev_compiler/lib/src/compiler/source_map_printer.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.
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_resolution_map.dart';
 import 'package:source_maps/source_maps.dart' hide Printer;
 import 'package:source_span/source_span.dart' show SourceLocation;
 
 import '../js_ast/js_ast.dart' as JS;
@@ skipping 58 matching lines @@
       _currentTopLevelDeclaration == null;
     }
   }
 
   // TODO(jmesserly): prefix identifiers too, if they map to a named element.
   String _getIdentifier(AstNode node) =>
       node is SimpleIdentifier ? node.name : null;
 
   void _mark(int offset, [String identifier]) {
     var loc = unit.lineInfo.getLocation(offset);
+    var next = unit.lineInfo.getLocation(offset + 1);

[vsm, 2017/04/11 18:39:51]

Can you add a comment here to explain?  This looks a bit magical.  :-)

[Alan Knight, 2017/04/11 20:32:06]

Done.

+    if (next.lineNumber == loc.lineNumber + 1) {
+      loc = next;
+    }
     sourceMap.addLocation(
         new SourceLocation(offset,
             sourceUrl: sourcePath,
             line: loc.lineNumber - 1,
             column: loc.columnNumber - 1),
         new SourceLocation(buffer.length, line: _line, column: _column),
         identifier);
   }
 }
 
 const int _LF = 10;
 const int _CR = 13;