Use explicit constructor in Draggable to fix IE10 null reference

There's a problem in IE10 with ES2015 classes and the super class.
Essentially the prototype isn't set up correctly which leads to
this.props being null when setting up position in the construction.

However, props are passed to the class constructor explicitly by React,
so we can use them from there.

Tested by running the examples in IE 10 on Win 7.

Author: davidstubbs

lib/Draggable.es6

@@ -18,6 +18,11 @@ type DraggableState = {
   isElementSVG: boolean
 };
 
+type ConstructorProps = {
+  position: { x: number, y: number },
+  defaultPosition: { x: number, y: number }
+}
+
 //
 // Define <Draggable>
 //
@@ -145,22 +150,28 @@ export default class Draggable extends React.Component {
     position: null
   };
 
-  state: DraggableState = {
-    // Whether or not we are currently dragging.
-    dragging: false,
+  state: DraggableState;
 
-    // Whether or not we have been dragged before.
-    dragged: false,
+  constructor(props: ConstructorProps) {
+    super(props);
+
+    this.state = {
+      // Whether or not we are currently dragging.
+      dragging: false,
 
-    // Current transform x and y.
-    x: this.props.position ? this.props.position.x : this.props.defaultPosition.x,
-    y: this.props.position ? this.props.position.y : this.props.defaultPosition.y,
+      // Whether or not we have been dragged before.
+      dragged: false,
 
-    // Used for compensating for out-of-bounds drags
-    slackX: 0, slackY: 0,
+      // Current transform x and y.
+      x: props.position ? props.position.x : props.defaultPosition.x,
+      y: props.position ? props.position.y : props.defaultPosition.y,
 
-    // Can only determine if SVG after mounting
-    isElementSVG: false
+      // Used for compensating for out-of-bounds drags
+      slackX: 0, slackY: 0,
+
+      // Can only determine if SVG after mounting
+      isElementSVG: false
+    };
   };
 
   componentWillMount() {
@@ -327,4 +338,3 @@ export default class Draggable extends React.Component {
     );
   }
 }
-