Fix some warnings.

frog/lib/string_implementation.dart

Index: frog/lib/string_implementation.dart
diff --git a/frog/lib/string_implementation.dart b/frog/lib/string_implementation.dart
index b42409e2418a8e4b9fe483d5aea5acc691e42097..45407ebe5964f1b81abad6a2efa15f49b9113254 100644
--- a/frog/lib/string_implementation.dart
+++ b/frog/lib/string_implementation.dart
@@ -91,6 +91,9 @@ return this.replace(from, to);""";
   List<String> _splitRegExp(RegExp pattern) native
     "'use strict'; return this.split(pattern.re);";
 
+  Iterable<Match> allMatches(String str) {
+    throw "String.allMatches(String str) unimplemented.";
+  }
   /*
   Iterable<Match> allMatches(String str) {
     List<Match> result = [];