This adds new methods to String object:

test/mjsunit/harmony/string-repeat.js

+// Copyright 2013 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+// Flags: --harmony-strings
+
+assertEquals("000", String.prototype.repeat.call(0, 3));
+assertEquals("-1-1-1", String.prototype.repeat.call(-1, 3));
+assertEquals("2.12.12.1", String.prototype.repeat.call(2.1, 3));
+assertEquals("", String.prototype.repeat.call([], 3));
+assertEquals("1,2,3", String.prototype.repeat.call([1, 2, 3], 1));
+assertEquals("true", String.prototype.repeat.call(true, 1));
+assertEquals("false", String.prototype.repeat.call(false, 1));
+assertEquals("[object Object]", String.prototype.repeat.call({}, 1));
+
+assertEquals("000", String.prototype.repeat.apply(0, [3]));
+assertEquals("-1-1-1", String.prototype.repeat.apply(-1, [3]));
+assertEquals("2.12.12.1", String.prototype.repeat.apply(2.1, [3]));
+assertEquals("", String.prototype.repeat.apply([], [3]));
+assertEquals("1,2,3", String.prototype.repeat.apply([1, 2, 3], [1]));
+assertEquals("true", String.prototype.repeat.apply(true, [1]));
+assertEquals("false", String.prototype.repeat.apply(false, [1]));
+assertEquals("[object Object]", String.prototype.repeat.apply({}, [1]));
+
+assertEquals("\u10D8\u10D8\u10D8", "\u10D8".repeat(3));
+
+assertThrows('String.prototype.repeat.call(null, 1)', TypeError);
+assertThrows('String.prototype.repeat.call(undefined, 1)', TypeError);
+assertThrows('String.prototype.repeat.apply(null, [1])', TypeError);
+assertThrows('String.prototype.repeat.apply(undefined, [1])', TypeError);
+
+// Test cases found in FF
+assertEquals("abc", "abc".repeat(1));
+assertEquals("abcabc", "abc".repeat(2));
+assertEquals("abcabcabc", "abc".repeat(3));
+assertEquals("aaaaaaaaaa", "a".repeat(10));
+assertEquals("", "".repeat(5));
+assertEquals("", "abc".repeat(0));
+assertEquals("abcabc", "abc".repeat(2.0));
+
+assertThrows('"a".repeat(-1)', RangeError);
+assertThrows('"a".repeat(Number.POSITIVE_INFINITY)', RangeError);
+
+var myobj = {
+  toString: function() {
+    return "abc";
+  },
+  repeat : String.prototype.repeat
+};
+assertEquals("abc", myobj.repeat(1));
+assertEquals("abcabc", myobj.repeat(2));