Fix d3 Selection.node and update test

tsc d3-test.ts passes

d3/d3-tests.ts

@@ -1960,7 +1960,7 @@ function healthAndWealth() {
             .text(function (d) { return d.name; } );
 
         // Add an overlay for the year label.
-        var box = label.node().getBBox();
+        var box = (<SVGTextElement>label.node()).getBBox();
         
         var overlay = svg.append("rect")
             .attr("class", "overlay")

d3/d3.d.ts

@@ -740,7 +740,7 @@ declare module D3 {
         */
         sort<T>(comparator?: (a: T, b: T) => number): Selection;
         order: () => Selection;
-        node: () => HTMLElement;
+        node: () => Element;
     }
 
     export interface EnterSelection {
@@ -748,7 +748,7 @@ declare module D3 {
         insert: (name: string, before: string) => Selection;
         select: (selector: string) => Selection;
         empty: () => boolean;
-        node: () => HTMLElement;
+        node: () => Element;
     }
 
     export interface UpdateSelection extends Selection {