Allow function as replacement argument to String.replaceAll.

runtime/lib/string_base.dart

Index: runtime/lib/string_base.dart
diff --git a/runtime/lib/string_base.dart b/runtime/lib/string_base.dart
index 11b594ed9e562cebc71e654acdb8b433678031f5..81b01c9f70e138c93a398a65aba570bc033c533d 100644
--- a/runtime/lib/string_base.dart
+++ b/runtime/lib/string_base.dart
@@ -210,12 +210,28 @@ class _StringBase {
     return buffer.add(this.substring(startIndex)).toString();
   }
 
-  String replaceAll(Pattern pattern, String replacement) {
+  String replaceAll(Pattern pattern, var replacement) {
     if (pattern is! Pattern) {
       throw new ArgumentError("${pattern} is not a Pattern");
     }
+    if (replacement is Function) {
+      StringBuffer buffer = new StringBuffer();
+      int startIndex = 0;
+      for (Match match in pattern.allMatches(this)) {
+        if (match.start > startIndex) {
+          buffer.add(this.substring(startIndex, match.start));
+        }
+        buffer.add(replacement(match).toString());
+        startIndex = match.end;
+      }
+      if (startIndex < this.length) {
+        buffer.add(this.substring(startIndex));
+      }
+      return buffer.toString();
+    }
     if (replacement is! String) {
-      throw new ArgumentError("${replacement} is not a String");
+      throw new ArgumentError(
+          "${replacement} is not a String or Match->String function");
     }
     StringBuffer buffer = new StringBuffer();
     int startIndex = 0;