Implement case mapping using the Unicode default case mapping algorithm.

runtime/lib/string.cc

Index: runtime/lib/string.cc
diff --git a/runtime/lib/string.cc b/runtime/lib/string.cc
index f6941d5e896ee35e60f4f5d6550727eb96c9c247..66d8513f92ca7411b1736cb42e9706ff9e30f4cd 100644
--- a/runtime/lib/string.cc
+++ b/runtime/lib/string.cc
@@ -140,6 +140,22 @@ DEFINE_NATIVE_ENTRY(String_concat, 2) {
 }
 
 
+DEFINE_NATIVE_ENTRY(String_toLowerCase, 1) {
+  const String& str = String::CheckedHandle(arguments->At(0));
+  ASSERT(!str.IsNull());
+  const String& result = String::Handle(String::ToLowerCase(str));
+  arguments->SetReturn(result);
+}
+
+
+DEFINE_NATIVE_ENTRY(String_toUpperCase, 1) {
+  const String& str = String::CheckedHandle(arguments->At(0));
+  ASSERT(!str.IsNull());
+  const String& result = String::Handle(String::ToUpperCase(str));
+  arguments->SetReturn(result);
+}
+
+
 DEFINE_NATIVE_ENTRY(Strings_concatAll, 1) {
   const Array& strings = Array::CheckedHandle(arguments->At(0));
   ASSERT(!strings.IsNull());