Add string.trimLeft/trimRight.

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 fce12492a1e44477ffd914a30119e9368fedbb73..5488a2ff6430d6c75a34ec03b3d8b29641791885 100644
--- a/sdk/lib/core/string.dart
+++ b/sdk/lib/core/string.dart
@@ -290,7 +290,7 @@ abstract class String implements Comparable<String>, Pattern {
   String substring(int startIndex, [int endIndex]);
 
   /**
-   * Removes leading and trailing whitespace from a string.
+   * Returns the string without any leading and trailing whitespace.
    *
    * If the string contains leading or trailing whitespace, a new string with no
    * leading and no trailing whitespace is returned:
@@ -326,6 +326,20 @@ abstract class String implements Comparable<String>, Pattern {
   String trim();
 
   /**
+   * Returns the string without any leading whitespace.
+   *
+   * As [trim], but only removes leading whitespace.
+   */
+  String trimLeft();
+
+  /**
+   * Returns the string without any trailing whitespace.
+   *
+   * As [trim], but only removes trailing whitespace.
+   */
+  String trimRight();
+
+  /**
    * Creates a new string by concatenating this string with itself a number
    * of times.
    *