Fix some bad documentation links.

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 4e69f65a4a86d182ed34163a6a8b99e55bc733b7..18ee8c3cc44f524f3e306ed7b5e19dd81719af9b 100644
--- a/sdk/lib/core/string.dart
+++ b/sdk/lib/core/string.dart
@@ -257,7 +257,7 @@ abstract class String implements Comparable<String>, Pattern {
    *
    *     string.indexOf(new RegExp(r'dart'));       // -1
    *
-   * [start] must not be negative or greater than [length].
+   * [start] must be non-negative and not greater than [length].
    */
   int indexOf(Pattern pattern, [int start]);
 
@@ -269,11 +269,11 @@ abstract class String implements Comparable<String>, Pattern {
    *     string.lastIndexOf('a');                    // 6
    *     string.lastIndexOf(new RegExp(r'a(r|n)'));  // 6
    *
-   * Returns -1 if [other] could not be found.
+   * Returns -1 if [pattern] could not be found in this string.
    *
    *     string.lastIndexOf(new RegExp(r'DART'));    // -1
    *
-   * [start] must not be negative or greater than [length].
+   * The [start] must be non-negative and not greater than [length].
    */
   int lastIndexOf(Pattern pattern, [int start]);
 
@@ -431,7 +431,7 @@ abstract class String implements Comparable<String>, Pattern {
    * Replace the first occurrence of [from] in this string.
    *
    * Returns a new string, which is this string
-   * except that the first match of [pattern], starting from [startIndex],
+   * except that the first match of [from], starting from [startIndex],
    * is replaced by the result of calling [replace] with the match object.
    *
    * The optional [startIndex] is by default set to 0. If provided, it must be