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

tests/compiler/dart2js/sourcemaps/diff.dart

 // Copyright (c) 2016, 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.diff;
 
 import 'package:compiler/src/io/source_file.dart';
 
 import 'html_parts.dart';
 import 'output_structure.dart';
@@ skipping 522 matching lines @@
   List<HtmlPart> codeLinesFromCodeSource(CodeSource codeSource) {
     List<HtmlPart> lines = <HtmlPart>[];
     SourceFile sourceFile = sourceFileManager.getSourceFile(codeSource.uri);
     String elementName = codeSource.name;
     HtmlLine line = new HtmlLine();
     line.htmlParts.add(new ConstHtmlPart('<span class="comment">'));
     line.htmlParts.add(new HtmlText('${elementName}: ${sourceFile.filename}'));
     line.htmlParts.add(new ConstHtmlPart('</span>'));
     lines.add(line);
     if (codeSource.begin != null) {
-      int startLine = sourceFile.getLine(codeSource.begin);
-      int endLine = sourceFile.getLine(codeSource.end) + 1;
+      int startLine = sourceFile.getLocation(codeSource.begin).line - 1;
+      int endLine = sourceFile.getLocation(codeSource.end).line;
       for (CodeLine codeLine in convertAnnotatedCodeToCodeLines(
           sourceFile.slowText(), const <Annotation>[],
           startLine: startLine, endLine: endLine)) {
         codeLine.lineAnnotation = codeSource;
         lines.add(codeLine);
       }
     }
     return lines;
   }
 
@@ skipping 30 matching lines @@
           startLine: interval.from, endLine: interval.to, uri: currentUri);
       if (codeSources != null) {
         CodeSource currentCodeSource;
         Interval currentLineInterval;
         for (CodeLine dartCodeLine in annotatedDartCodeLines) {
           if (currentCodeSource == null ||
               !currentLineInterval.contains(dartCodeLine.lineNo)) {
             currentCodeSource = null;
             for (CodeSource codeSource in codeSources) {
               Interval interval = new Interval(
-                  sourceFile.getLine(codeSource.begin),
-                  sourceFile.getLine(codeSource.end) + 1);
+                  sourceFile.getLocation(codeSource.begin).line - 1,
+                  sourceFile.getLocation(codeSource.end).line);
               if (interval.contains(dartCodeLine.lineNo)) {
                 currentCodeSource = codeSource;
                 currentLineInterval = interval;
                 break;
               }
             }
           }
           if (currentCodeSource != null) {
             dartCodeLine.lineAnnotation = currentCodeSource;
           }
@@ skipping 25 matching lines @@
       codeLineInterval.clear();
       codeLineInterval[codeLine] = interval;
     }
 
     for (CodeLine jsCodeLine in jsCodeLines) {
       for (Annotation annotation in jsCodeLine.annotations) {
         CodeLineAnnotation codeLineAnnotation = annotation.data;
 
         for (CodeLocation location in codeLineAnnotation.codeLocations) {
           SourceFile sourceFile = sourceFileManager.getSourceFile(location.uri);
-          int line = sourceFile.getLine(location.offset);
+          int line = sourceFile.getLocation(location.offset).line - 1;
           if (currentUri != location.uri) {
             restart(jsCodeLine, location, line);
           } else if (interval.inWindow(line, windowSize: 2)) {
             interval = interval.include(line);
             codeLineInterval[jsCodeLine] = interval;
           } else {
             restart(jsCodeLine, location, line);
           }
 
           annotations.add(new Annotation(codeLineAnnotation.annotationType,
@@ skipping 106 matching lines @@
         annotationType: AnnotationType.values[json['annotationType']],
         codeLocations: json['codeLocations']
             .map((j) => CodeLocation.fromJson(j, strategy))
             .toList(),
         codeSources:
             json['codeSources'].map((j) => CodeSource.fromJson(j)).toList(),
         stepInfo: json['stepInfo'],
         sourceMappingIndex: json['sourceMappingIndex']);
   }
 }