Allow setting unknown CSS properties, e.g. CSS variables

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index d87ca4f80d137b320e4353c1a6d38b7594ad2cd7..0ad6f36e686dcb8ca5707affc6969a3266b221d4 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -6188,15 +6188,11 @@ class CssStyleDeclaration extends DartHtmlDomObject
   /// Please note the property name uses camelCase, not-hyphens.
   String getPropertyValue(String propertyName) {
     var propValue = _getPropertyValueHelper(propertyName);
-    return propValue != null ? propValue : '';
+    return propValue ?? '';
   }
 
   String _getPropertyValueHelper(String propertyName) {
-    if (_supportsProperty(_camelCase(propertyName))) {
-      return _getPropertyValue(propertyName);
-    } else {
-      return _getPropertyValue(Device.cssPrefix + propertyName);
-    }
+    return _getPropertyValue(_browserPropertyName(propertyName));
   }
 
   /**
@@ -6209,7 +6205,7 @@ class CssStyleDeclaration extends DartHtmlDomObject
    */
   bool supportsProperty(String propertyName) {
     return _supportsProperty(propertyName) ||
-        _supportsProperty(_camelCase(Device.cssPrefix + propertyName));
+        _supportsProperty(_camelCase("${Device.cssPrefix}$propertyName"));
   }
 
   bool _supportsProperty(String propertyName) {
@@ -6234,15 +6230,23 @@ class CssStyleDeclaration extends DartHtmlDomObject
   String _browserPropertyName(String propertyName) {
     String name = _readCache(propertyName);
     if (name is String) return name;
-    if (_supportsProperty(_camelCase(propertyName))) {
-      name = propertyName;
-    } else {
-      name = Device.cssPrefix + propertyName;
-    }
+    name = _supportedBrowserPropertyName(propertyName);
     _writeCache(propertyName, name);
     return name;
   }
 
+  String _supportedBrowserPropertyName(String propertyName) {
+    if (_supportsProperty(_camelCase(propertyName))) {
+      return propertyName;
+    }
+    var prefixed = "${Device.cssPrefix}$propertyName";
+    if (_supportsProperty(prefixed)) {
+      return prefixed;
+    }
+    // May be a CSS variable, just use it as provided.
+    return propertyName;
+  }
+
   static String _readCache(String key) => null;
   static void _writeCache(String key, value) {}