Convert String to a class.

lib/compiler/implementation/lib/string_helper.dart

Index: lib/compiler/implementation/lib/string_helper.dart
diff --git a/lib/compiler/implementation/lib/string_helper.dart b/lib/compiler/implementation/lib/string_helper.dart
index a67e784c6de3496ec1a20c193ad48a14d2a22693..6556bcc7744b861fda4b8fdcc8163f5631a70de4 100644
--- a/lib/compiler/implementation/lib/string_helper.dart
+++ b/lib/compiler/implementation/lib/string_helper.dart
@@ -106,7 +106,7 @@ stringReplaceAllUnchecked(receiver, from, to) {
   } else {
     checkNull(from);
     // TODO(floitsch): implement generic String.replace (with patterns).
-    throw "StringImplementation.replaceAll(Pattern) UNIMPLEMENTED";
+    throw "String.replaceAll(Pattern) UNIMPLEMENTED";
   }
 }
 
@@ -119,7 +119,7 @@ stringReplaceFirstUnchecked(receiver, from, to) {
   } else {
     checkNull(from);
     // TODO(floitsch): implement generic String.replace (with patterns).
-    throw "StringImplementation.replace(Pattern) UNIMPLEMENTED";
+    throw "String.replace(Pattern) UNIMPLEMENTED";
   }
 }
 
@@ -130,7 +130,7 @@ stringSplitUnchecked(receiver, pattern) {
     var re = regExpGetNative(pattern);
     return JS('List', r'#.split(#)', receiver, re);
   } else {
-    throw "StringImplementation.split(Pattern) UNIMPLEMENTED";
+    throw "String.split(Pattern) UNIMPLEMENTED";
   }
 }