Change VM's string-buffer patch to use a Uin16Array as backing buffer.

sdk/lib/_internal/compiler/implementation/lib/core_patch.dart

Index: sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart b/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
index 1da20f6e90c14bd70e32cbfd216d45c2efffb1d7..fc2765b6f583603e5bd4cab881d6807d642f510b 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
@@ -234,6 +234,10 @@ patch class StringBuffer {
     _contents = Primitives.stringConcatUnchecked(_contents, str);
   }
 
+  patch void writeCharCode(int charCode) {
+    write(new String.fromCharCode(charCode));
+  }
+
   patch void clear() {
     _contents = "";
   }