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

tests/compiler/dart2js/source_map_validator_helper.dart

 // Copyright (c) 2014, 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 'dart:io';
 import 'dart:convert';
 import 'dart:async';
 
 import 'package:path/path.dart' as path;
 import 'package:expect/expect.dart';
 import 'package:source_maps/source_maps.dart';
 import 'package:compiler/src/apiimpl.dart';
 import 'package:compiler/src/elements/elements.dart'
     show
         AstElement,
         ClassElement,
         CompilationUnitElement,
         Element,
         FunctionElement,
         LibraryElement,
         MemberElement;
 import 'package:compiler/src/io/source_file.dart' show SourceFile;
 import 'package:compiler/src/io/source_information.dart'
     show computeElementNameForSourceMaps;
+import 'package:kernel/ast.dart' show Location;
 
 validateSourceMap(Uri targetUri,
     {Uri mainUri, Position mainPosition, CompilerImpl compiler}) {
   Uri mapUri = getMapUri(targetUri);
   List<String> targetLines = new File.fromUri(targetUri).readAsLinesSync();
   SingleMapping sourceMap = getSourceMap(mapUri);
   checkFileReferences(targetUri, mapUri, sourceMap);
   checkIndexReferences(targetLines, mapUri, sourceMap);
   checkRedundancy(sourceMap);
   if (compiler != null) {
@@ skipping 90 matching lines @@
             new Position(entry.sourceLine, entry.sourceColumn);
         String name = sourceMap.names[entry.sourceNameId];
 
         CompilationUnitElement compilationUnit = compilationUnitMap[uri];
         Expect.isNotNull(
             compilationUnit, "No compilation unit found for $uri.");
 
         SourceFile sourceFile = compilationUnit.script.file;
 
         Position positionFromOffset(int offset) {
-          int line = sourceFile.getLine(offset);
-          int column = sourceFile.getColumn(line, offset);
+          Location location = sourceFile.getLocation(offset);
+          int line = location.line - 1;
+          int column = location.column - 1;
           return new Position(line, column);
         }
 
         Interval intervalFromElement(AstElement element) {
           if (!element.hasNode) return null;
 
           var begin = element.node.getBeginToken().charOffset;
           var end = element.node.getEndToken();
           end = end.charOffset + end.charCount;
           return new Interval(
@@ skipping 168 matching lines @@
   final Position end;
 
   Interval(this.begin, this.end);
 
   bool contains(Position other) {
     return begin <= other && other <= end;
   }
 
   String toString() => '$begin-$end';
 }