Remove deprecated string features.

tests/language/string_test.dart

Index: tests/language/string_test.dart
diff --git a/tests/language/string_test.dart b/tests/language/string_test.dart
index 52cdf71ab98b5b61be401c12e3e760e2ea783bae..546274b226b5bf71f755fab00180a1cea9cf2ee0 100644
--- a/tests/language/string_test.dart
+++ b/tests/language/string_test.dart
@@ -16,7 +16,7 @@ class StringTest {
     String str = "string";
     for (int i = 0; i < str.length; i++) {
       Expect.equals(true, str[i] is String);
-      Expect.equals(true, str.charCodeAt(i) is int);
+      Expect.equals(true, str.codeUnitAt(i) is int);
     }
   }
 
@@ -42,7 +42,7 @@ class StringTest {
   static testCharCodes() {
     String s = new String.fromCharCodes(const [0x41, 0xC1, 0x424]);
     Expect.equals("A", s[0]);
-    Expect.equals(0x424, s.charCodeAt(2));
+    Expect.equals(0x424, s.codeUnitAt(2));
   }
 }