fix(datepicker): solve refocusing problems

* Fixes the datepicker not being able to refocus on the calendar icon, because the SVG element inside didn't propagate the focus event.
* Fixes the datepicker sometimes not refocusing on the element that triggered it.

Fixes #8960.

Closes #9080

Author: crisbeto

src/components/datepicker/datePicker.scss

@@ -33,6 +33,19 @@ md-datepicker {
   box-sizing: border-box;
   background: none;
   vertical-align: middle;
+  position: relative;
+
+  // Captures any of the click events. This is necessary, because the button has a SVG
+  // icon which doesn't propagate the focus event, causing inconsistent behaviour.
+  &:before {
+    top: 0;
+    left: 0;
+    bottom: 0;
+    right: 0;
+    position: absolute;
+    content: '';
+    speak: none;
+  }
 }
 
 // The input into which the user can type the date.

src/components/datepicker/js/datepickerDirective.js

@@ -682,28 +682,29 @@
     if (this.isCalendarOpen) {
       var self = this;
 
+      self.detachCalendarPane();
+      self.ngModelCtrl.$setTouched();
+
+      self.documentElement.off('click touchstart', self.bodyClickHandler);
+      window.removeEventListener('resize', self.windowResizeHandler);
+
       self.calendarPaneOpenedFrom.focus();
       self.calendarPaneOpenedFrom = null;
 
       if (self.openOnFocus) {
-        // Ensures that all focus events have fired before detaching
+        // Ensures that all focus events have fired before resetting
         // the calendar. Prevents the calendar from reopening immediately
         // in IE when md-open-on-focus is set. Also it needs to trigger
         // a digest, in order to prevent issues where the calendar wasn't
         // showing up on the next open.
-        this.$mdUtil.nextTick(detach);
+        self.$mdUtil.nextTick(reset);
       } else {
-        detach();
+        reset();
       }
     }
 
-    function detach() {
-      self.detachCalendarPane();
+    function reset(){
       self.isCalendarOpen = self.isOpen = false;
-      self.ngModelCtrl.$setTouched();
-
-      self.documentElement.off('click touchstart', self.bodyClickHandler);
-      window.removeEventListener('resize', self.windowResizeHandler);
     }
   };