Updated flex CSS prefix for Firefox.

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 565e521b2abee091f5ba1af846f8ab2a27b12e65..a3e3f9b6ffc047c4ba32be1ddfb18bac08d71aa4 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -2555,6 +2555,7 @@ class CanvasRenderingContext2D extends CanvasRenderingContext {
   /** Deprecated always returns 1.0 */
   @DomName('CanvasRenderingContext2D.webkitBackingStorePixelRation')
   @Experimental()
+  @deprecated
   double get backingStorePixelRatio => 1.0;
 }
 
@@ -4634,12 +4635,17 @@ abstract class CssStyleDeclarationBase {
   }
 
   /** Gets the value of "flex" */
-  String get flex =>
-    getPropertyValue('${Device.cssPrefix}flex');
+  String get flex {
+    String prefix = Device.cssPrefix
+    if (Device.isFirefox) prefix = '';
+    return getPropertyValue('${prefix}flex');
+  }
 
   /** Sets the value of "flex" */
   void set flex(String value) {
-    setProperty('${Device.cssPrefix}flex', value, '');
+    String prefix = Device.cssPrefix
+    if (Device.isFirefox) prefix = '';
+    setProperty('${prefix}flex', value, '');
   }
 
   /** Gets the value of "flex-basis" */