diff --git a/java/google/registry/ui/js/session.js b/java/google/registry/ui/js/session.js
index f2f1358ca..5aa47c1a7 100644
--- a/java/google/registry/ui/js/session.js
+++ b/java/google/registry/ui/js/session.js
@@ -48,13 +48,14 @@ registry.Session = function(defaultUri, xsrfToken, contentType) {
 
   /**
    * XHR request headers.
-   * @private {!Map.<string, string>}
+   * @private {!Object<string, string>}
    * @const
    */
-  this.headers_ = new Map([
-      ['Content-Type', contentType],
-      ['X-CSRF-Token', xsrfToken],
-      ['X-Requested-With', 'XMLHttpRequest']]);
+  this.headers_ = {
+    'Content-Type': contentType,
+    'X-CSRF-Token': xsrfToken,
+    'X-Requested-With': 'XMLHttpRequest'
+  };
 };
 
 
diff --git a/javatests/google/registry/ui/js/registrar/console_test.js b/javatests/google/registry/ui/js/registrar/console_test.js
index 88ced3203..fd3a346e4 100644
--- a/javatests/google/registry/ui/js/registrar/console_test.js
+++ b/javatests/google/registry/ui/js/registrar/console_test.js
@@ -135,7 +135,7 @@ function testNavToResources() {
   assertTrue(xhr.isActive());
   assertEquals('/registrar-settings', xhr.getLastUri());
   assertEquals(test.testXsrfToken,
-               xhr.getLastRequestHeaders().get('X-CSRF-Token'));
+               xhr.getLastRequestHeaders()['X-CSRF-Token']);
   xhr.simulateResponse(200, goog.json.serialize({
     status: 'SUCCESS',
     message: 'OK',
@@ -161,7 +161,7 @@ function testNavToContactUs() {
   assertTrue(xhr.isActive());
   assertEquals('/registrar-settings', xhr.getLastUri());
   assertEquals(test.testXsrfToken,
-               xhr.getLastRequestHeaders().get('X-CSRF-Token'));
+               xhr.getLastRequestHeaders()['X-CSRF-Token']);
   var passcode = '5-5-5-5-5';
   xhr.simulateResponse(200, goog.json.serialize({
     status: 'SUCCESS',
diff --git a/javatests/google/registry/ui/js/registrar/domain_test.js b/javatests/google/registry/ui/js/registrar/domain_test.js
index 16d3dc01b..44e10d6a8 100644
--- a/javatests/google/registry/ui/js/registrar/domain_test.js
+++ b/javatests/google/registry/ui/js/registrar/domain_test.js
@@ -117,7 +117,7 @@ function handleLogin() {
   var xhr = goog.testing.net.XhrIo.getSendInstances().pop();
   assertTrue(xhr.isActive());
   assertEquals('/registrar-xhr', xhr.getLastUri());
-  assertEquals('☢', xhr.getLastRequestHeaders().get('X-CSRF-Token'));
+  assertEquals('☢', xhr.getLastRequestHeaders()['X-CSRF-Token']);
   registry.testing.assertXmlEquals(request, xhr.getLastContent());
   xhr.simulateResponse(200, response);
 }
@@ -183,7 +183,7 @@ function testView() {
   var xhr = goog.testing.net.XhrIo.getSendInstances().pop();
   assertTrue('XHR is inactive.', xhr.isActive());
   assertEquals('/registrar-xhr', xhr.getLastUri());
-  assertEquals('☢', xhr.getLastRequestHeaders().get('X-CSRF-Token'));
+  assertEquals('☢', xhr.getLastRequestHeaders()['X-CSRF-Token']);
   registry.testing.assertXmlEquals(request, xhr.getLastContent());
   xhr.simulateResponse(200, response);
   assertEquals('We require more vespene gas.',
@@ -252,7 +252,7 @@ function testEdit() {
   var xhr = goog.testing.net.XhrIo.getSendInstances().pop();
   assertTrue('XHR is inactive.', xhr.isActive());
   assertEquals('/registrar-xhr', xhr.getLastUri());
-  assertEquals('☢', xhr.getLastRequestHeaders().get('X-CSRF-Token'));
+  assertEquals('☢', xhr.getLastRequestHeaders()['X-CSRF-Token']);
   registry.testing.assertXmlEquals(request, xhr.getLastContent());
   xhr.simulateResponse(200, response);
 
@@ -307,7 +307,7 @@ function testEdit() {
   xhr = goog.testing.net.XhrIo.getSendInstances().pop();
   assertTrue('XHR is inactive.', xhr.isActive());
   assertEquals('/registrar-xhr', xhr.getLastUri());
-  assertEquals('☢', xhr.getLastRequestHeaders().get('X-CSRF-Token'));
+  assertEquals('☢', xhr.getLastRequestHeaders()['X-CSRF-Token']);
   registry.testing.assertXmlEquals(request, xhr.getLastContent());
   xhr.simulateResponse(200, response);
   assertEquals('We require more vespene gas.',
@@ -414,7 +414,7 @@ function testCreate() {
   var xhr = goog.testing.net.XhrIo.getSendInstances().pop();
   assertTrue('XHR is inactive.', xhr.isActive());
   assertEquals('/registrar-xhr', xhr.getLastUri());
-  assertEquals('☢', xhr.getLastRequestHeaders().get('X-CSRF-Token'));
+  assertEquals('☢', xhr.getLastRequestHeaders()['X-CSRF-Token']);
   registry.testing.assertXmlEquals(request, xhr.getLastContent());
   xhr.simulateResponse(200, response);
 
@@ -469,7 +469,7 @@ function testCreate() {
   xhr = goog.testing.net.XhrIo.getSendInstances().pop();
   assertTrue('XHR is inactive.', xhr.isActive());
   assertEquals('/registrar-xhr', xhr.getLastUri());
-  assertEquals('☢', xhr.getLastRequestHeaders().get('X-CSRF-Token'));
+  assertEquals('☢', xhr.getLastRequestHeaders()['X-CSRF-Token']);
   registry.testing.assertXmlEquals(request, xhr.getLastContent());
   xhr.simulateResponse(200, response);
   assertEquals('We require more vespene gas.',
diff --git a/javatests/google/registry/ui/js/registrar/host_test.js b/javatests/google/registry/ui/js/registrar/host_test.js
index c369621dd..0ed64350e 100644
--- a/javatests/google/registry/ui/js/registrar/host_test.js
+++ b/javatests/google/registry/ui/js/registrar/host_test.js
@@ -117,7 +117,7 @@ function handleLogin() {
   var xhr = goog.testing.net.XhrIo.getSendInstances().pop();
   assertTrue(xhr.isActive());
   assertEquals('/registrar-xhr', xhr.getLastUri());
-  assertEquals('☢', xhr.getLastRequestHeaders().get('X-CSRF-Token'));
+  assertEquals('☢', xhr.getLastRequestHeaders()['X-CSRF-Token']);
   registry.testing.assertXmlEquals(request, xhr.getLastContent());
   xhr.simulateResponse(200, response);
 }
@@ -174,8 +174,8 @@ function testView() {
   assertTrue('XHR is inactive.', xhr.isActive());
   assertEquals('/registrar-xhr', xhr.getLastUri());
   assertEquals('application/epp+xml',
-               xhr.getLastRequestHeaders().get('Content-Type'));
-  assertEquals('☢', xhr.getLastRequestHeaders().get('X-CSRF-Token'));
+               xhr.getLastRequestHeaders()['Content-Type']);
+  assertEquals('☢', xhr.getLastRequestHeaders()['X-CSRF-Token']);
   registry.testing.assertXmlEquals(request, xhr.getLastContent());
   xhr.simulateResponse(200, response);
   assertEquals('We require more vespene gas.',
@@ -242,7 +242,7 @@ function testEditFirstAddr_ignoreSecond_addThird() {
   var xhr = goog.testing.net.XhrIo.getSendInstances().pop();
   assertTrue('XHR is inactive.', xhr.isActive());
   assertEquals('/registrar-xhr', xhr.getLastUri());
-  assertEquals('☢', xhr.getLastRequestHeaders().get('X-CSRF-Token'));
+  assertEquals('☢', xhr.getLastRequestHeaders()['X-CSRF-Token']);
   registry.testing.assertXmlEquals(request, xhr.getLastContent());
   xhr.simulateResponse(200, response);
 
@@ -288,7 +288,7 @@ function testEditFirstAddr_ignoreSecond_addThird() {
   xhr = goog.testing.net.XhrIo.getSendInstances().pop();
   assertTrue('XHR is inactive.', xhr.isActive());
   assertEquals('/registrar-xhr', xhr.getLastUri());
-  assertEquals('☢', xhr.getLastRequestHeaders().get('X-CSRF-Token'));
+  assertEquals('☢', xhr.getLastRequestHeaders()['X-CSRF-Token']);
   registry.testing.assertXmlEquals(request, xhr.getLastContent());
   xhr.simulateResponse(200, response);
   assertEquals('We require more vespene gas.',
@@ -364,7 +364,7 @@ function testCreate() {
   var xhr = goog.testing.net.XhrIo.getSendInstances().pop();
   assertTrue('XHR is inactive.', xhr.isActive());
   assertEquals('/registrar-xhr', xhr.getLastUri());
-  assertEquals('☢', xhr.getLastRequestHeaders().get('X-CSRF-Token'));
+  assertEquals('☢', xhr.getLastRequestHeaders()['X-CSRF-Token']);
   registry.testing.assertXmlEquals(request, xhr.getLastContent());
   xhr.simulateResponse(200, response);
 
@@ -413,7 +413,7 @@ function testCreate() {
   xhr = goog.testing.net.XhrIo.getSendInstances().pop();
   assertTrue('XHR is inactive.', xhr.isActive());
   assertEquals('/registrar-xhr', xhr.getLastUri());
-  assertEquals('☢', xhr.getLastRequestHeaders().get('X-CSRF-Token'));
+  assertEquals('☢', xhr.getLastRequestHeaders()['X-CSRF-Token']);
   registry.testing.assertXmlEquals(request, xhr.getLastContent());
   xhr.simulateResponse(200, response);
   assertEquals('We require more vespene gas.',