Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

node_modules/cssstyle/lib/properties/flexShrink.js

@@ -0,0 +1,19 @@
+'use strict';
+
+var parseNumber = require('../parsers').parseNumber;
+var POSITION_AT_SHORTHAND = require('../constants').POSITION_AT_SHORTHAND;
+
+module.exports.isValid = function isValid(v, positionAtFlexShorthand) {
+  return parseNumber(v) !== undefined && positionAtFlexShorthand === POSITION_AT_SHORTHAND.second;
+};
+
+module.exports.definition = {
+  set: function(v) {
+    this._setProperty('flex-shrink', parseNumber(v));
+  },
+  get: function() {
+    return this.getPropertyValue('flex-shrink');
+  },
+  enumerable: true,
+  configurable: true,
+};