Add String.repeat, String.padLeft, String.padRight.

sdk/lib/core/string.dart

Index: sdk/lib/core/string.dart
diff --git a/sdk/lib/core/string.dart b/sdk/lib/core/string.dart
index 799a2a96837e06652a54cb6cfb6b243d1ebea063..12f6790dc50ffad8ff712cf5b3c0a252f892a6b1 100644
--- a/sdk/lib/core/string.dart
+++ b/sdk/lib/core/string.dart
@@ -326,6 +326,78 @@ abstract class String implements Comparable<String>, Pattern {
   String trim();
 
   /**
+   * Creates a new string that repeats this string a number of times.
+   *
+   * If [separator] is provided, it is insert between the copies

[floitsch, 2014/02/19 10:20:36]

it is inserted

[Lasse Reichstein Nielsen, 2014/02/19 11:43:16]

Done.

+   * of this string.
+   *
+   * The [times] number must be non-negative.
+   */
+  String repeat(int times, [String separator = ""]);

[floitsch, 2014/02/19 10:20:36]

Do we really need this one?

The only good use-case I can think of (that isn't handled by padLeft/padRight)
would be several new lines...

[Lasse Reichstein Nielsen, 2014/02/19 11:43:16]

If we have padLeft, you can do str.repeat(n) by doing "".padLeft(n, str). I'd
rather have a dedicated repeat than have to watch that hack.
I do believe repeating a string is useful in general.

+
+  /**
+   * Pads this string on the left if it is shorther than [newSize].
+   *
+   * For each position the length of this string is shorter than [newSize],
+   * a copy of [padding] is added on the left.
+   * If the length of this string is already greater than [newSize],
+   * this string is returned unmodified.
+   *
+   * If [padding] has length greater than 1,
+   * the resulting string's length will be greater than [newSize].
+   * This can be used for cases where the logical length of a string
+   * is not its length in code points.
+   * Example:
+   *
+   *     "foo".padLeft(10, " ");      // String representing single space.
+   *     "foo".padLeft(10, "\u{100002}");  // Surrogate pair character.
+   *
+   * If [size] is provided, it is used as instead of `this.length`
+   * as the size of this string.
+   * If the string contains, for example, surrogate pairs,
+   * and it is intended to be treated as charaters, not code points,
+   * [size] can be used to provide the logical length.
+   * Example:
+   *
+   *     "D&D".padLeft(10, " ", size: 3);
+   *
+   * The [size] number must be non-negative.
+   * If [newSize] is negative, no padding will happen.
+   */
+  String padLeft(int newSize, String padding, { int size });

[floitsch, 2014/02/19 10:20:36]

I'm not convinced that dealing with surrogate pairs is necessary. In Utf-16 they
are very rare, and once you deal with non-standard unicode characters you
probably don't deal with strings of specific sizes anymore. (Why would you want
to pad with three spaces, if the runes in your string merge together...)

Most of the time padding will be " ".

I would have had the following signature:
String padLeft(int newSize, [String padding = " "])

I would also require the padding to be of size 1.

All other uses can easily written by hand.

[Søren Gjesse, 2014/02/19 10:44:10]

I find the option of padding with size larger than one useful. Regarding the
names size argument actually using this requires some way of getting the actual
"rendering length", which we don't really have except for .runes.length - which
does not handle combining characters.

[Lasse Reichstein Nielsen, 2014/02/19 11:43:16]

This is not (just) dealing with surrogate pairs. In fact, it doesn't deal with
surrogates pairs at all. They are used as examples in the same way as the HTML
entities.
What it does do is leaving an opening for the user to deal with strings that
have a logical size different from their physical length.

The padding should generally have a logical size of 1, but that doesn't mean it
must be a string of length 1. The " " example isn't mine, it's from the
feature request.

From looking at our own code, padding is usually '0' or ' ', probably with '0'
being slightly more common.

+
+  /**
+   * Pads this string on the right if it is shorther than [newSize].
+   *
+   * For each position the length of this string is shorter than [newSize],
+   * a copy of [padding] is added on the right.
+   * If the length of this string is already greater than [newSize],
+   * this string is returned unmodified.
+   *
+   * If [padding] has length greater than 1,
+   * the resulting string's length will be greater than [newSize].
+   * This can be used for cases where the logical length of a string
+   * is not its length in code points.
+   * Example:
+   *
+   *     "foo".padRight(10, " ");      // String representing single space.
+   *     "foo".padRight(10, "\u{100002}");  // Surrogate pair character.
+   *
+   * If [size] is provided, it is used as instead of `this.length`
+   * as the size of this string.
+   * If the string contains, for example, surrogate pairs,
+   * and it is intended to be treated as charaters, not code points,
+   * [size] can be used to provide the logical length.
+   * Example:
+   *
+   *     "D&D".padLeft(10, " ", size: 3);
+   *
+   * The [size] number must be non-negative.
+   * If [newSize] is negative, no padding will happen.
+   */
+  String padRight(int newSize, String padding, {int size});
+
+  /**
    * Returns true if this string contains a match of [other]:
    *
    *     var string = 'Dart strings';