feat(datepicker): Add invalid class for datepicker

Add a red border to the bottom of the datepicker input if the input
string is not a valid and complete date.

src/components/datepicker/datePicker-theme.scss

@@ -8,6 +8,10 @@ md-datepicker.md-THEME_NAME-theme {
 
   .md-datepicker-input-container {
     border-bottom-color: '{{background-300}}';
+
+    &.md-datepicker-invalid {
+      border-bottom-color: '{{warn-500}}';
+    }
   }
 
   .md-datepicker-calendar-pane {

src/components/datepicker/datePicker.js

@@ -9,7 +9,6 @@
   // TODO(jelbourn): make sure this plays well with validation and ngMessages.
   // TODO(jelbourn): calendar pane doesn't open up outside of visible viewport.
   // TODO(jelbourn): forward more attributes to the internal input (required, autofocus, etc.)
-  // TODO(jelbourn): error state
   // TODO(jelbourn): something better for mobile (calendar panel takes up entire screen?)
   // TODO(jelbourn): input behavior (masking? auto-complete?)
   // TODO(jelbourn): UTC mode
@@ -86,6 +85,9 @@
   /** Additional offset for the input's `size` attribute, which is updated based on its content. */
   var EXTRA_INPUT_SIZE = 3;
 
+  /** Class applied to the container if the date is invalid. */
+  var INVALID_CLASS = 'md-datepicker-invalid';
+
   /**
    * Controller for md-datepicker.
    *
@@ -214,7 +216,11 @@
 
       if (self.dateUtil.isValidDate(parsedDate) && self.dateLocale.isDateComplete(inputString)) {
         self.ngModelCtrl.$setViewValue(parsedDate);
+        self.inputContainer.classList.remove(INVALID_CLASS);
         self.$scope.$apply();
+      } else {
+        // If there's an input string, it's an invalid date.
+        self.inputContainer.classList.toggle(INVALID_CLASS, inputString);
       }
     });
   };