Override CSSStyleDeclaration.getPropertyValue in general way.

lib/html/idl/dart/dart.idl

 
 // This file introduces / supplements and forces Dart declarations.
 
 module default {
   // TODO(efortuna): Clean up these comments once we have dealt with the extra
   // methods on these list-like classes.
   ClientRectList implements sequence<ClientRect>;
   CSSRuleList implements sequence<CSSRule>;
   CSSValueList implements sequence<CSSValue>;
   DOMMimeTypeArray implements sequence<DOMMimeType>; // has namedItem field.
@@ skipping 184 matching lines @@
 
     any getUniform(in WebGLProgram program, in WebGLUniformLocation location) raises(DOMException);
     [Suppressed, StrictTypeChecking, Custom] void getUniform();
 
     any getVertexAttrib(in unsigned long index, in unsigned long pname) raises(DOMException);
     [Suppressed, StrictTypeChecking, Custom] void getVertexAttrib();
   };
 }
 
 module css {
+  [Supplemental]
   interface CSSStyleDeclaration {
     void setProperty(in DOMString propertyName, in DOMString value, [Optional] in DOMString priority);
+    [DartName=_getPropertyValue] DOMString getPropertyValue(in DOMString propertyName);
   };
 };
 
 module canvas {
   [Supplemental,
    Constructor(in long length)]  // Add constructor signature.
   interface ArrayBuffer {
   };
   [Supplemental]
   interface Float32Array {
@@ skipping 185 matching lines @@
 }
 
 module core {
   [supplemental, Callback]
   interface RequestAnimationFrameCallback {
     // Webkit implements this as returning bool, but standard is void.
     [Suppressed] boolean handleEvent(in DOMTimeStamp time);
     [Custom] void handleEvent(in DOMTimeStamp time);
   };
 }