fix(slider): fix a js error if the slider container doesn't have a md-input-container

* Fixes the slider trying to call getComputedStyle if it doesn't have child md-input-container.
* Uses a slightly faster way of finding the child md-input-container.

Fixes #8174, #7728.

Closes #8225

Author: crisbeto

src/components/slider/slider.js

@@ -70,19 +70,22 @@ function SliderContainerDirective() {
         var initialMaxWidth;
 
         ctrl.fitInputWidthToTextLength = function (length) {
-          var input = element.find('md-input-container');
-          var computedStyle = getComputedStyle(input[0]);
-          var minWidth = parseInt(computedStyle['min-width']);
-          var padding = parseInt(computedStyle['padding']) * 2;
-          initialMaxWidth = initialMaxWidth || parseInt(computedStyle['max-width']);
+          var input = element[0].querySelector('md-input-container');
 
-          var newMaxWidth = Math.max(initialMaxWidth, minWidth + padding + (minWidth / 2 * length));
+          if (input) {
+            var computedStyle = getComputedStyle(input);
+            var minWidth = parseInt(computedStyle.minWidth);
+            var padding = parseInt(computedStyle.padding) * 2;
 
-          input.css('max-width', newMaxWidth + 'px');
+            initialMaxWidth = initialMaxWidth || parseInt(computedStyle.maxWidth);
+            var newMaxWidth = Math.max(initialMaxWidth, minWidth + padding + (minWidth / 2 * length));
+
+            input.style.maxWidth = newMaxWidth + 'px';
+          }
         };
-      }
+      };
     }
-  }
+  };
 }
 
 /**