Don't show dart2js package warnings.

lib/src/runner/browser/compiler_pool.dart

Index: lib/src/runner/browser/compiler_pool.dart
diff --git a/lib/src/runner/browser/compiler_pool.dart b/lib/src/runner/browser/compiler_pool.dart
index edc41bf20dd41f7191d43ea6518273063be29d6d..54f51ea733a3ae94972ca0ac806e65fe4c8f2f09 100644
--- a/lib/src/runner/browser/compiler_pool.dart
+++ b/lib/src/runner/browser/compiler_pool.dart
@@ -71,7 +71,7 @@ void main(_) {
         var dart2jsPath = p.join(sdkDir, 'bin', 'dart2js');
         if (Platform.isWindows) dart2jsPath += '.bat';
 
-        var args = ["--checked", wrapperPath, "--out=$jsPath", "--show-package-warnings"];
+        var args = ["--checked", wrapperPath, "--out=$jsPath"];
 
         if (packageRoot != null) {
           args.add("--package-root=${p.toUri(p.absolute(packageRoot))}");