Add test

web/lib/src/helpers/extensions.dart

@@ -104,6 +104,12 @@ extension TouchListConvert on TouchList {
   List<Touch> toList() => JSImmutableListWrapper<TouchList, Touch>(this);
 }
 
+/// Returns all response headers as a key-value map.
+///
+/// Multiple values for the same header key can be combined into one,
+/// separated by a comma and a space.
+///
+/// See: http://www.w3.org/TR/XMLHttpRequest/#the-getresponseheader()-method
 extension XMLHttpRequestGlue on XMLHttpRequest {
   Map<String, String> get responseHeaders {
     // from Closure's goog.net.Xhrio.getResponseHeaders.

web/test/helpers_test.dart

@@ -36,4 +36,22 @@ void main() {
     // Ensure accessing any arbitrary item in the list does not throw.
     expect(() => dartList[0], returnsNormally);
   });
+
+  test('Headers to map', () async {
+    final request = XMLHttpRequest();
+    request.open('GET', 'www.google.com');
+    request.send();
+    await request.onLoad.first;
+
+    expect(
+      request.responseHeaders,
+      allOf(
+        containsPair('content-length', '10'),
+        containsPair('content-type', 'text/plain; charset=utf-8'),
+        containsPair('x-content-type-options', 'nosniff'),
+        containsPair('x-frame-options', 'SAMEORIGIN'),
+        containsPair('x-xss-protection', '1; mode=block'),
+      ),
+    );
+  });
 }