Create and test source mapping for invocations.

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

 // Copyright (c) 2012, 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.
 
 library dart2js.io.line_column;
 
 import 'code_output.dart';
 
 /// Interface for providing line/column information.
 abstract class LineColumnProvider {
   /// Returns the line number (0-based) for [offset].
   int getLine(int offset);
 
   /// Returns the column number (0-based) for [offset] at the given [line].
   int getColumn(int line, int offset);
+
+  /// Returns the offset for 0-based [line] and [column] numbers.
+  int getOffset(int line, int column);
 }
 
 /// [CodeOutputListener] that collects line information.
 class LineColumnCollector extends CodeOutputListener
     implements LineColumnProvider {
   int length = 0;
   List<int> lineStarts = <int>[0];
 
   void _collect(String text) {
     int index = 0;
@@ skipping 32 matching lines @@
       }
     }
     return first;
   }
 
   @override
   int getColumn(int line, int offset) {
     return offset - lineStarts[line];
   }
 
+  int getOffset(int line, int column) => lineStarts[line] + column;
+
   @override
   void onDone(int length) {
     lineStarts.add(length + 1);
     this.length = length;
   }
 
   String toString() {
     return 'lineStarts=$lineStarts,length=$length';
   }
 }