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

tests/compiler/dart2js/sourcemaps/sourcemap_helper.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.
 
 library sourcemap.helper;
 
 import 'dart:async';
 import 'dart:io';
 import 'package:compiler/compiler_new.dart';
 import 'package:compiler/src/apiimpl.dart' as api;
 import 'package:compiler/src/commandline_options.dart';
 import 'package:compiler/src/null_compiler_output.dart' show NullSink;
 import 'package:compiler/src/elements/elements.dart';
 import 'package:compiler/src/helpers/helpers.dart';
 import 'package:compiler/src/filenames.dart';
 import 'package:compiler/src/io/code_output.dart';
 import 'package:compiler/src/io/source_file.dart';
 import 'package:compiler/src/io/source_information.dart';
 import 'package:compiler/src/io/position_information.dart';
 import 'package:compiler/src/js/js.dart' as js;
 import 'package:compiler/src/js/js_debug.dart';
 import 'package:compiler/src/js/js_source_mapping.dart';
 import 'package:compiler/src/js_backend/js_backend.dart';
 import 'package:compiler/src/source_file_provider.dart';
+import 'package:kernel/ast.dart' show Location;
 import '../memory_compiler.dart';
 import '../output_collector.dart';
 
 class SourceFileSink implements OutputSink {
   final String filename;
   StringBuffer sb = new StringBuffer();
   SourceFile sourceFile;
 
   SourceFileSink(this.filename);
 
@@ skipping 458 matching lines @@
     register(kind, node);
   }
 
   void register(StepKind kind, js.Node node, {bool expectInfo: true}) {
     String dartCodeFromSourceLocation(SourceLocation sourceLocation) {
       SourceFile sourceFile =
           sourceFileManager.getSourceFile(sourceLocation.sourceUri);
       if (sourceFile == null) {
         return sourceLocation.shortText;
       }
-      return sourceFile
-          .getLineText(sourceLocation.line)
+      return sourceFile.kernelSource
+          .getTextLine(sourceLocation.line + 1)
           .substring(sourceLocation.column)
           .trim();
     }
 
     void addLocation(SourceLocation sourceLocation, String jsCode) {
       if (sourceLocation == null) {
         if (expectInfo) {
           SourceInformation sourceInformation = node.sourceInformation;
           SourceLocation sourceLocation;
           String dartCode;
@@ skipping 54 matching lines @@
       absoluteUri = base.resolveUri(uri);
     } else {
       absoluteUri = base.resolve(uri);
     }
     return sourceFiles.putIfAbsent(absoluteUri, () {
       String text = new File.fromUri(absoluteUri).readAsStringSync();
       return new StringSourceFile.fromUri(absoluteUri, text);
     });
   }
 }