Add String.replaceRange and use it in replaceFirst{,Mapped}.

sdk/lib/_internal/compiler/js_lib/js_string.dart

Index: sdk/lib/_internal/compiler/js_lib/js_string.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/js_string.dart b/sdk/lib/_internal/compiler/js_lib/js_string.dart
index e682626f8d7200b197e511320b2d27497687ed22..3ba213ad66586217e4da62e3e15b9f1bd522f196 100644
--- a/sdk/lib/_internal/compiler/js_lib/js_string.dart
+++ b/sdk/lib/_internal/compiler/js_lib/js_string.dart
@@ -97,6 +97,16 @@ class JSString extends Interceptor implements String, JSIndexable {
     }
   }
 
+  String replaceRange(String replacement, int start, [int end]) {
+    checkString(replacement);
+    checkInt(start);
+    end = RangeError.checkValidRange(start, end, this.length);
+    checkInt(end);
+    var prefix = JS('String', '#.substring(0, #)', this, start);
+    var suffix = JS('String', '#.substring(#)', this, end);
+    return "$prefix$replacement$suffix";

[sra1, 2015/02/23 18:58:27]

There are use cases for an unchecked version.

replace these three lines with:
return stringReplaceRangeUnchecked(this, start, end, replacement)
Add stringReplaceRangeUnchecked to string_helper.js

There are a couple of places in string_helper that could then call
stringReplaceRangeUnchecked, e.g. stringReplaceFirstUnchecked.

The cases that follow from String or RegExp are safe.
The cases that follow from a general Pattern might not be safe.

[Lasse Reichstein Nielsen, 2015/02/25 12:09:09]

Done.

+  }
+
   List<String> _defaultSplit(Pattern pattern) {
     List<String> result = <String>[];
     // End of most recent match. That is, start of next part to add to result.