dart2js isn't strong mode clean yet.

pkg/testing/lib/src/analyze.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.md file.
 
 library testing.analyze;
 
 import 'dart:async' show Stream, Future;
 
 import 'dart:convert' show LineSplitter, UTF8;
 
@@ skipping 219 matching lines @@
   process.stdin.writeln(arguments.join(" "));
   process.stdin.close();
 
   bool hasOutput = false;
   Set<String> seen = new Set<String>();
 
   processAnalyzerOutput(Stream<AnalyzerDiagnostic> diagnostics) async {
     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;
       }
       String message = "$diagnostic";
       if (seen.add(message)) {
         hasOutput = true;
         print(message);
       }
     }
@@ skipping 18 matching lines @@
   ProcessResult result = await Process.run(
       Platform.isWindows ? "git.bat" : "git",
       <String>[command]..addAll(arguments),
       workingDirectory: workingDirectory);
   if (result.exitCode != 0) {
     throw "Non-zero exit code from git $command (${result.exitCode})\n"
         "${result.stdout}\n${result.stderr}";
   }
   return result.stdout;
 }