Add startsWith/endsWith from sugar.js trunk

ui/base.js

@@ -1,4 +1,60 @@
 
+// Borrowed from sugar.js HEAD since startsWith/endsWith were very slow when
+// they supported regexps.
+(function() {
+    function getCoercedStringSubject(obj) {
+        if(obj == null) {
+            throw new TypeError();
+        }
+        return String(obj);
+    }
+
+    function getCoercedSearchString(obj, errorOnRegex) {
+        if(errorOnRegex && obj instanceof RegExp) {
+            throw new TypeError();
+        }
+        return String(obj);
+    }
+
+    String.prototype.startsWith = function(searchString) {
+        var str, start, pos, len, searchLength, position = arguments[1];
+        str = getCoercedStringSubject(this);
+        searchString = getCoercedSearchString(searchString, true);
+        pos = Number(position) || 0;
+        len = str.length;
+        start = Math.min(Math.max(pos, 0), len);
+        searchLength = searchString.length;
+        if(searchLength + start > len) {
+            return false;
+        }
+        if(str.substr(start, searchLength) === searchString) {
+            return true;
+        }
+        return false;
+    };
+
+    String.prototype.endsWith = function(searchString) {
+        var str, start, end, pos, len, searchLength, endPosition = arguments[1];
+        str = getCoercedStringSubject(this);
+        searchString = getCoercedSearchString(searchString, true);
+        len = str.length;
+        pos = len;
+        if(endPosition != undefined) {
+            pos = Number(endPosition) || 0;
+        }
+        end = Math.min(Math.max(pos, 0), len);
+        searchLength = searchString.length;
+        start = end - searchLength;
+        if(start < 0) {
+            return false;
+        }
+        if(str.substr(start, searchLength) === searchString) {
+            return true;
+        }
+        return false;
+    };
+})();
+
 // Polymer provides this, but we add it just in case so you can use models
 // without polymer.
 if (!NodeList.prototype.array) {