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

tests/compiler/dart2js/sourcemaps/sourcemap_helper.dart

Index: tests/compiler/dart2js/sourcemaps/sourcemap_helper.dart
diff --git a/tests/compiler/dart2js/sourcemaps/sourcemap_helper.dart b/tests/compiler/dart2js/sourcemaps/sourcemap_helper.dart
index 85a19e012f2870d040838453e251dde945bd087c..f5c02ef34fc2b5978b723766cee3b1d2e8d712c8 100644
--- a/tests/compiler/dart2js/sourcemaps/sourcemap_helper.dart
+++ b/tests/compiler/dart2js/sourcemaps/sourcemap_helper.dart
@@ -22,6 +22,7 @@ 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';
 
@@ -500,8 +501,8 @@ class CodePointComputer extends TraceListener {
       if (sourceFile == null) {
         return sourceLocation.shortText;
       }
-      return sourceFile
-          .getLineText(sourceLocation.line)
+      return sourceFile.kernelSource
+          .getTextLine(sourceLocation.line + 1)
           .substring(sourceLocation.column)
           .trim();
     }