Remove deprecated string features.

sdk/lib/_internal/compiler/implementation/lib/string_helper.dart

Index: sdk/lib/_internal/compiler/implementation/lib/string_helper.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/string_helper.dart b/sdk/lib/_internal/compiler/implementation/lib/string_helper.dart
index 974902831e51dffbe396216b003b5f9992c0069c..8f3c2fe2f9da083f12b06702db6cefb307670dde 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/string_helper.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/string_helper.dart
@@ -145,10 +145,10 @@ stringReplaceAllEmptyFuncUnchecked(receiver, onMatch, onNonMatch) {
   while (i < length) {
     buffer.add(onMatch(new StringMatch(i, receiver, "")));
     // Special case to avoid splitting a surrogate pair.
-    int code = receiver.charCodeAt(i);
+    int code = receiver.codeUnitAt(i);
     if ((code & ~0x3FF) == 0xD800 && length > i + 1) {
       // Leading surrogate;
-      code = receiver.charCodeAt(i + 1);
+      code = receiver.codeUnitAt(i + 1);
       if ((code & ~0x3FF) == 0xDC00) {
         // Matching trailing surrogate.
         buffer.add(onNonMatch(receiver.substring(i, i + 2)));