dart2js isn't strong mode clean yet.

pkg/testing/lib/src/analyze.dart

Index: pkg/testing/lib/src/analyze.dart
diff --git a/pkg/testing/lib/src/analyze.dart b/pkg/testing/lib/src/analyze.dart
index 296a023fb58389f25878366eed2a12bb1a550e31..8719df746a90c69c5b4097c9aedb69639e5db9b5 100644
--- a/pkg/testing/lib/src/analyze.dart
+++ b/pkg/testing/lib/src/analyze.dart
@@ -237,10 +237,12 @@ Future<Null> analyzeUris(
     await for (AnalyzerDiagnostic diagnostic in diagnostics) {
       if (diagnostic.uri != null) {
         String path = toFilePath(diagnostic.uri);
-        if (diagnostic.code.startsWith("STRONG_MODE") &&
+        if (!(analysisOptions?.path.contains("/pkg/compiler/") ?? false) &&
+            diagnostic.code.startsWith("STRONG_MODE") &&
             (path.startsWith("pkg/compiler/") ||
                 path.startsWith("tests/compiler/dart2js/"))) {
-          // Hack to work around dart2js not being strong-mode clean.
+          // TODO(ahe): Remove this hack to work around dart2js not being
+          // strong-mode clean.
           continue;
         }
         if (!filesToAnalyze.contains(path)) continue;