Add white-listing to sourcemap tests.

tests/compiler/dart2js/sourcemaps/sourcemap_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.
 
 library sourcemap.helper;
 
 import 'dart:async';
 import 'dart:io';
 import 'package:compiler/compiler_new.dart';
 import 'package:compiler/src/apiimpl.dart' as api;
@@ skipping 487 matching lines @@
     int nl = line.indexOf('\n');
     if (nl != -1) {
       line = line.substring(0, nl);
     }
     return line;
   }
 
   /// Called when [node] defines a step of the given [kind] at the given
   /// [offset] when the generated JavaScript code.
   void onStep(js.Node node, Offset offset, StepKind kind) {
-    register('$kind', node);
+    register(kind, node);
   }
 
-  void register(String kind, js.Node node, {bool expectInfo: true}) {
+  void register(StepKind kind, js.Node node, {bool expectInfo: true}) {
 
     String dartCodeFromSourceLocation(SourceLocation sourceLocation) {
       SourceFile sourceFile =
            sourceFileManager.getSourceFile(sourceLocation.sourceUri);
       if (sourceFile == null) {
         return sourceLocation.shortText;
       }
       return sourceFile.getLineText(sourceLocation.line)
           .substring(sourceLocation.column).trim();
     }
@@ skipping 26 matching lines @@
         for (SourceLocation location in locations) {
           addLocation(location, jsCode);
         }
       });
     }
   }
 }
 
 /// A JavaScript code point and its mapped dart source location.
 class CodePoint {
-  final String kind;
+  final StepKind kind;
   final String jsCode;
   final SourceLocation sourceLocation;
   final String dartCode;
   final bool isMissing;
 
   CodePoint(
       this.kind,
       this.jsCode,
       this.sourceLocation,
       this.dartCode,
@@ skipping 18 matching lines @@
       absoluteUri = base.resolveUri(uri);
     } else {
       absoluteUri = base.resolve(uri);
     }
     return sourceFiles.putIfAbsent(absoluteUri, () {
       String text = new File.fromUri(absoluteUri).readAsStringSync();
       return new StringSourceFile.fromUri(absoluteUri, text);
     });
   }
 }