Support nullable attributes and return types in Dartium.

tools/dom/idl/dart/dart.idl

 // This file introduces / supplements and forces Dart declarations.
 
 [Supplemental,
  Constructor]
 interface AudioContext {};
 
 [Supplemental]
 interface Document {
   [Suppressed] DOMObject getCSSCanvasContext(in DOMString contextId, in DOMString name, in long width, in long height);
   CanvasRenderingContext getCSSCanvasContext(in DOMString contextId, in DOMString name, in long width, in long height);
@@ skipping 24 matching lines @@
 interface CanvasRenderingContext2D {
   [DartName=createImageDataFromImageData] ImageData createImageData(in ImageData imagedata) raises (DOMException);
 };
 
 [Supplemental]
 interface Console {
   [Suppressed] void assert(in boolean condition);
   [CallWith=ScriptArguments|ScriptState] void assertCondition(boolean condition);
 };
 
-// TODO(antonm): remove those overrides and corresponding custom methods.
-[Supplemental]
-interface Coordinates {
-    [Custom] readonly attribute double altitude;
-    [Custom] readonly attribute double altitudeAccuracy;
-    [Custom] readonly attribute double heading;
-    [Custom] readonly attribute double speed;
-};
-
-
 interface HTMLCanvasElement {
   [Suppressed] DOMString toDataURL([TreatNullAs=NullString, TreatUndefinedAs=NullString,Optional=DefaultIsUndefined] in DOMString type) raises(DOMException);
   [Custom] DOMString toDataURL([TreatNullAs=NullString, TreatUndefinedAs=NullString,Optional=DefaultIsUndefined] in DOMString type, [Optional] in float quality) raises(DOMException);
 
   [Suppressed] any getContext(in DOMString contextId);
   [Custom] CanvasRenderingContext getContext(in DOMString contextId, [Optional] in Dictionary attrs);
 };
 
 [Supplemental]
 interface HTMLOptionsCollection {
@@ skipping 294 matching lines @@
        raises(DOMException);
    [Custom] void send([StrictTypeChecking] in DOMString data)
        raises(DOMException);
    [Custom] void send(in DOMFormData data)
        raises(DOMException);
 };
 
 
 [Suppressed]
 interface Entity {};