Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/sourcemaps/html_parts.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.html_parts;
 
 import 'sourcemap_html_helper.dart';
 
 class Annotation {
   final id;
@@ skipping 144 matching lines @@
           return ConstHtmlPart.fromJson(json, strategy);
         case HtmlPartKind.NEWLINE:
           return const NewLine();
         case HtmlPartKind.TEXT:
           return HtmlText.fromJson(json, strategy);
         case HtmlPartKind.TAG:
           return TagPart.fromJson(json, strategy);
         case HtmlPartKind.LINE_NUMBER:
           return LineNumber.fromJson(json, strategy);
       }
+      return null;
     }
   }
 }
 
 class ConstHtmlPart implements HtmlPart {
   final String html;
 
   const ConstHtmlPart(this.html);
 
   HtmlPartKind get kind => HtmlPartKind.CONST;
@@ skipping 137 matching lines @@
       AnnotationData annotationDataForSpan =
           context.getAnnotationData(included, forSpan: true);
 
       String head = subsequentCode;
       String tail = '';
       if (subsequentCode.length > 1) {
         head = subsequentCode.substring(0, 1);
         tail = subsequentCode.substring(1);
       }
 
-      void addForSpan(AnnotationData data) {
-        htmlParts.add(new TagPart(data.tag,
-            properties: data.properties,
-            content: [new HtmlText(subsequentCode)]));
-      }
-
       if (annotationData != null && annotationDataForSpan != null) {
         htmlParts.add(new TagPart(annotationDataForSpan.tag,
             properties: annotationDataForSpan.properties,
             content: [
               new TagPart(annotationData.tag,
                   properties: annotationData.properties,
                   content: [new HtmlText(head)]),
               new HtmlText(tail)
             ]));
       } else if (annotationDataForSpan != null) {
@@ skipping 144 matching lines @@
 
   Annotation decodeAnnotation(Map json) {
     return new Annotation(json['id'], json['codeOffset'], json['title'],
         data: json['data']);
   }
 
   encodeLineAnnotation(lineAnnotation) => lineAnnotation;
 
   decodeLineAnnotation(json) => json;
 }