This adds new methods to String object:

src/harmony-string.js

Index: src/harmony-string.js
diff --git a/src/harmony-string.js b/src/harmony-string.js
new file mode 100644
index 0000000000000000000000000000000000000000..701079aa76b2d65ce4a7e97583eb43bd563c34e1
--- /dev/null
+++ b/src/harmony-string.js
@@ -0,0 +1,151 @@
+// 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.
+
+'use strict';
+
+// This file relies on the fact that the following declaration has been made
+// in runtime.js:
+// var $String = global.String;
+// var $Array = global.Array;
+

[Michael Starzinger, 2013/07/30 10:58:39]

nit: Please add the separation marker as present in other builtin JS files here.

+// ES6 draft 07-15-13, section 15.5.3.21
+function StringRepeat(count) {
+  if (IS_NULL_OR_UNDEFINED(this) && !IS_UNDETECTABLE(this)) {
+    throw MakeTypeError("called_on_null_or_undefined",
+                        ["String.prototype.repeat"]);
+  }
+
+  var s = TO_STRING_INLINE(this);
+  var n = ToInteger(count);
+  if (n < 0 || !NUMBER_IS_FINITE(n)) {
+    throw MakeRangeError("invalid_count_value", []);
+  }
+
+  var elements = new InternalArray(n);
+  var i;

[Michael Starzinger, 2013/07/30 10:58:39]

nit: Please move the variable declaration of "i" into the for-statement (i.e.
"for (var i = ...").

+  for (i = 0; i < n; i++) {
+    elements[i] = s;
+  }
+
+  return %StringBuilderConcat(elements, n, "");
+}
+
+
+// ES6 draft 07-15-13, section 15.5.3.22
+function StringStartsWith(searchString /* position */) {  // length == 1
+  if (IS_NULL_OR_UNDEFINED(this) && !IS_UNDETECTABLE(this)) {
+    throw MakeTypeError("called_on_null_or_undefined",
+                        ["String.prototype.startsWith"]);
+  }
+
+  var s = TO_STRING_INLINE(this);
+  var ss = TO_STRING_INLINE(searchString);
+  var pos = 0;
+  if (%_ArgumentsLength() > 1) {
+    pos = %_Arguments(1);  // position
+    pos = ToInteger(pos);
+  }
+
+  var s_len = s.length;
+  var start = MathMin(MathMax(pos, 0), s_len);
+  var ss_len = ss.length;
+  if (ss_len + start > s_len) {
+    return false;
+  }
+
+  return %StringIndexOf(s, ss, start) === start;
+}
+
+
+// ES6 draft 07-15-13, section 15.5.3.23
+function StringEndsWith(searchString /* position */) {  // length == 1
+  if (IS_NULL_OR_UNDEFINED(this) && !IS_UNDETECTABLE(this)) {
+    throw MakeTypeError("called_on_null_or_undefined",
+                        ["String.prototype.endsWith"]);
+  }
+
+  var s = TO_STRING_INLINE(this);
+  var ss = TO_STRING_INLINE(searchString);
+  var s_len = s.length;
+  var pos = s_len;
+  if (%_ArgumentsLength() > 1) {
+    var arg = %_Arguments(1);  // position
+    if (!IS_UNDEFINED(arg)) {
+      pos = ToInteger(arg);
+    }
+  }
+
+  var end = MathMin(MathMax(pos, 0), s_len);
+  var ss_len = ss.length;
+  var start = end - ss_len;
+  if (start < 0) {
+    return false;
+  }
+
+  return %StringLastIndexOf(s, ss, start) === end - ss_len;

[Michael Starzinger, 2013/07/30 10:58:39]

nit: s/end - ss_len/start/ here, makes it easier to read.

+}
+
+
+// ES6 draft 07-15-13, section 15.5.3.24
+function StringContains(searchString /* position */) {  // length == 1
+  if (IS_NULL_OR_UNDEFINED(this) && !IS_UNDETECTABLE(this)) {
+    throw MakeTypeError("called_on_null_or_undefined",
+                        ["String.prototype.contains"]);
+  }
+
+  var s = TO_STRING_INLINE(this);
+  var ss = TO_STRING_INLINE(searchString);
+  var pos = 0;
+  if (%_ArgumentsLength() > 1) {
+    pos = %_Arguments(1);  // position
+    pos = ToInteger(pos);
+  }
+
+  var s_len = s.length;
+  var start = MathMin(MathMax(pos, 0), s_len);
+  var ss_len = ss.length;
+  if (ss_len + start > s_len) {
+    return false;
+  }
+
+  return %StringIndexOf(s, ss, start) !== -1;
+}
+
+

[Michael Starzinger, 2013/07/30 10:58:39]

nit: Please add the separation marker as present in other builtin JS files here.

+function ExtendStringPrototype() {
+  %CheckIsBootstrapping();
+
+  // Set up the non-enumerable functions on the String prototype object.
+  InstallFunctions($String.prototype, DONT_ENUM, $Array(
+    "repeat", StringRepeat,
+    "startsWith", StringStartsWith,
+    "endsWith", StringEndsWith,
+    "contains", StringContains
+  ));
+}
+
+ExtendStringPrototype();