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

tests/compiler/dart2js/sourcemaps/sourcemap_html_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.
 
 /// Helper for creating HTML visualization of the source map information
 /// generated by a [SourceMapProcessor].
 
 library sourcemap.html.helper;
 
 import 'dart:convert';
@@ skipping 497 matching lines @@
         dartCodeBuffer.write('<h4>${uri.pathSegments.last}, '
             '${firstLineIndex - windowSize + 1}-'
             '${lastLineIndex + windowSize + 1}'
             '</h4>\n');
         dartCodeBuffer.write('<pre>\n');
         for (int line = firstLineIndex - windowSize;
             line < firstLineIndex;
             line++) {
           if (line >= 0) {
             dartCodeBuffer.write(lineNumber(line, width: lineNoWidth));
-            dartCodeBuffer.write(sourceFile.getLineText(line));
+            dartCodeBuffer.write(sourceFile.kernelSource.getTextLine(line + 1));
           }
         }
         dartCodeBuffer.write(codeBuffer);
         for (int line = lastLineIndex + 1;
             line <= lastLineIndex + windowSize;
             line++) {
           if (line < sourceFile.lines) {
             dartCodeBuffer.write(lineNumber(line, width: lineNoWidth));
-            dartCodeBuffer.write(sourceFile.getLineText(line));
+            dartCodeBuffer.write(sourceFile.kernelSource.getTextLine(line + 1));
           }
         }
         dartCodeBuffer.write('</pre>\n');
         firstLineIndex = null;
         lastLineIndex = null;
       }
       codeBuffer.clear();
     }
 
     lineIndices.forEach((int lineIndex) {
       List<SourceLocation> locations = uriMap[lineIndex];
       if (lastLineIndex != null && lastLineIndex + windowSize * 4 < lineIndex) {
         flush();
       }
       if (firstLineIndex == null) {
         firstLineIndex = lineIndex;
       } else {
         for (int line = lastLineIndex + 1; line < lineIndex; line++) {
           codeBuffer.write(lineNumber(line, width: lineNoWidth));
-          codeBuffer.write(sourceFile.getLineText(line));
+          codeBuffer.write(sourceFile.kernelSource.getTextLine(line + 1));
         }
       }
-      String line = sourceFile.getLineText(lineIndex);
+      String line = sourceFile.kernelSource.getTextLine(lineIndex + 1);
       locations.sort((a, b) => a.offset.compareTo(b.offset));
       for (int i = 0; i < locations.length; i++) {
         SourceLocation sourceLocation = locations[i];
         int index = collection.getIndex(sourceLocation);
         int start = sourceLocation.column;
         int end = line.length;
         if (i + 1 < locations.length) {
           end = locations[i + 1].column;
         }
         if (i == 0) {
@@ skipping 59 matching lines @@
       String dartCode = truncate(codePoint.dartCode, 50);
       buffer.write('<td class="code">${dartCode}</td>');
       buffer.write('<td>${escape(codePoint.sourceLocation.shortText)}</td>');
     }
     buffer.write('</tr>');
   });
   buffer.write('</table>');
 
   return buffer.toString();
 }