Adapt String interface for Utf16.

runtime/lib/string_base.dart

Index: runtime/lib/string_base.dart
diff --git a/runtime/lib/string_base.dart b/runtime/lib/string_base.dart
index 1f6d9fe7bfd28d5fc6cb0b00bd53f13943978722..39f4518eb159c77dfe0456bb266d1d0fad211137 100644
--- a/runtime/lib/string_base.dart
+++ b/runtime/lib/string_base.dart
@@ -38,7 +38,11 @@ class _StringBase {
 
   String operator [](int index) native "String_charAt";
 
-  int charCodeAt(int index) native "String_charCodeAt";
+  int charCodeAt(int index) {
+    return codeUnitAt(index);
+  }
+
+  int codeUnitAt(int index) native "String_charCodeAt";
 
   int get length native "String_getLength";
 
@@ -429,6 +433,14 @@ class _StringBase {
     return result;
   }
 
+  Iterable<int> get codeUnits {
+    throw new UnimplementedError("String.codeUnits");
+  }
+
+  Iterable<int> get runes {
+    throw new UnimplementedError("String.runes");
+  }
+
   String toUpperCase() native "String_toUpperCase";
 
   String toLowerCase() native "String_toLowerCase";