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

tests/compiler/dart2js/sourcemaps/diff_view.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_view;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
@@ skipping 834 matching lines @@
       nextSourceMappedLocationIndex++;
     }
   }
 
   // Associate JavaScript offsets with [Element]s.
   StringSourceFile sourceFile = new StringSourceFile.fromName(filename, code);
   Map<int, Element> elementMap = <int, Element>{};
   sourceMaps.elementSourceMapInfos
       .forEach((Element element, SourceMapInfo info) {
     CodePosition position = info.jsCodePositions[info.node];
-    elementMap[sourceFile.getLine(position.startPosition)] = element;
+    elementMap[sourceFile.getLocation(position.startPosition).line - 1] =
+        element;
   });
 
   codeLines = convertAnnotatedCodeToCodeLines(code, annotations);
   return new CodeLinesResult(codeLines, coverage, elementMap,
       sourceMaps.sourceFileManager, codeSources);
 }
 
 /// Visitor that computes a map from [js.Node]s to all attached source
 /// locations.
 class SourceLocationCollector extends js.BaseVisitor {
@@ skipping 89 matching lines @@
     if (data.annotationType == AnnotationType.WITHOUT_SOURCE_INFO) {
       return new AnnotationData(tag: 'span', properties: {
         'title': annotation.title,
         'class': '${ClassNames.marker} '
             '${data.annotationType.className}'
       });
     }
   }
   return null;
 }