Override CSSStyleDeclaration.getPropertyValue in general way.

lib/html/dartium/html_dartium.dart

Index: lib/html/dartium/html_dartium.dart
diff --git a/lib/html/dartium/html_dartium.dart b/lib/html/dartium/html_dartium.dart
index 129715eae1eee1981852ec036d4d449860367493..c30975559e1c298dc27406b48518c447e4dab8d6 100644
--- a/lib/html/dartium/html_dartium.dart
+++ b/lib/html/dartium/html_dartium.dart
@@ -2929,9 +2929,6 @@ abstract class CSSStyleDeclaration  {
   /** @domName CSSStyleDeclaration.getPropertyShorthand */
   String getPropertyShorthand(String propertyName);
 
-  /** @domName CSSStyleDeclaration.getPropertyValue */
-  String getPropertyValue(String propertyName);
-
   /** @domName CSSStyleDeclaration.isPropertyImplicit */
   bool isPropertyImplicit(String propertyName);
 
@@ -3731,6 +3728,9 @@ abstract class CSSStyleDeclaration  {
   /** Sets the value of "font-weight" */
   void set fontWeight(var value);
 
+  /** @domName CSSStyleDeclaration.getPropertyValue. */
+  String getPropertyValue(String propertyName);
+
   /** Gets the value of "height" */
   String get height;
 
@@ -4825,7 +4825,7 @@ class _CSSStyleDeclarationImpl extends NativeFieldWrapperClass1 implements CSSSt
 
   String getPropertyShorthand(String propertyName) native "CSSStyleDeclaration_getPropertyShorthand_Callback";
 
-  String getPropertyValue(String propertyName) native "CSSStyleDeclaration_getPropertyValue_Callback";
+  String _getPropertyValue(String propertyName) native "CSSStyleDeclaration__getPropertyValue_Callback";
 
   bool isPropertyImplicit(String propertyName) native "CSSStyleDeclaration_isPropertyImplicit_Callback";
 
@@ -4836,6 +4836,11 @@ class _CSSStyleDeclarationImpl extends NativeFieldWrapperClass1 implements CSSSt
   void setProperty(String propertyName, String value, [String priority]) native "CSSStyleDeclaration_setProperty_Callback";
 
 
+  String getPropertyValue(String propertyName) {
+    var propValue = _getPropertyValue(propertyName);
+    return propValue != null ? propValue : '';
+  }
+
 
   // TODO(jacobr): generate this list of properties using the existing script.
     /** Gets the value of "animation" */