Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/sourcemaps/html_parts.dart

Index: tests/compiler/dart2js/sourcemaps/html_parts.dart
diff --git a/tests/compiler/dart2js/sourcemaps/html_parts.dart b/tests/compiler/dart2js/sourcemaps/html_parts.dart
index 2691a0919d2406c67c60cdc5e9fe44e7379c6173..d02e3e53ba1b6cd5465ef5e8daa7fb616d5d7466 100644
--- a/tests/compiler/dart2js/sourcemaps/html_parts.dart
+++ b/tests/compiler/dart2js/sourcemaps/html_parts.dart
@@ -162,6 +162,7 @@ abstract class HtmlPart {
         case HtmlPartKind.LINE_NUMBER:
           return LineNumber.fromJson(json, strategy);
       }
+      return null;
     }
   }
 }
@@ -319,12 +320,6 @@ class CodePart {
         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,