Add Source.getTextLine and use it to display source snippets in error messages.

tests/compiler/dart2js/warnings_checker.dart

 // Copyright (c) 2013, 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.
 
 // Test that dart2js produces the expected static type warnings to ensures that
 // the analyzer and dart2js agrees on the tests.
 
 import 'dart:async';
 import 'dart:io';
 import 'package:expect/expect.dart';
@@ skipping 41 matching lines @@
         if (statusMap != null && statusMap.containsKey('unexpected')) {
           unexpectedStatus = statusMap['unexpected'];
         }
         // Line numbers with known missing warnings.
         List<int> missingStatus = [];
         if (statusMap != null && statusMap.containsKey('missing')) {
           missingStatus = statusMap['missing'];
         }
         for (CollectedMessage message in collector.warnings) {
           Expect.equals(uri, message.uri);
-          int lineNo = file.getLine(message.begin);
+          int lineNo = file.getLocation(message.begin).line - 1;
           if (expectedWarnings.containsKey(lineNo)) {
             unseenWarnings.remove(lineNo);
           } else if (!unexpectedStatus.contains(lineNo + 1)) {
             warningsMismatch = true;
             print(file.getLocationMessage(
                 'Unexpected warning: ${message.message}',
                 message.begin,
                 message.end));
           }
         }
         if (!unseenWarnings.isEmpty) {
           for (int lineNo in unseenWarnings) {
             if (!missingStatus.contains(lineNo + 1)) {
               warningsMismatch = true;
               String line = expectedWarnings[lineNo];
               print('$uri [${lineNo+1}]: Missing static type warning.');
               print(line);
             }
           }
         }
       }).then((_) {
         Expect.isFalse(warningsMismatch);
       }));
 }