Improve performance of dart2js StringBuffer.

dart/tests/language/string_interpolation_and_buffer.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // Test to ensure that StringBuffer and string interpolation behaves
 // the same and fail fast.
 
 class ToStringWrapper {
   final value;
 
@@ skipping 38 matching lines @@
       try {
         sb = new StringBuffer().add(wrap(object));
       } on ArgumentError {
         return 'Error';
       }
       Expect.isTrue(sb.toString() is String);
     }
     return 'Success';
   }
 
+  initBuffer(object) {
+    var sb;
+    if (checkedMode && object != null) {
+      try {
+        sb = new StringBuffer(wrap(object));
+      } on TypeError {
+        return 'Error';
+      }
+    } else {
+      try {
+        sb = new StringBuffer(wrap(object));
+      } on ArgumentError {
+        return 'Error';
+      }
+      Expect.isTrue(sb.toString() is String);
+    }
+    return 'Success';
+  }
+
   Expect.equals('Error', interpolate(null));
   Expect.equals('Success', interpolate(""));
   Expect.equals('Success', interpolate("string"));
   Expect.equals('Error', interpolate([]));
   Expect.equals('Error', interpolate([1]));
   Expect.equals('Error', interpolate(new Object()));
 
   Expect.equals('Error', buffer(null));
   Expect.equals('Success', buffer(""));
   Expect.equals('Success', buffer("string"));
   Expect.equals('Error', buffer([]));
   Expect.equals('Error', buffer([1]));
   Expect.equals('Error', buffer(new Object()));
+
+  Expect.equals('Error', initBuffer(null));
+  Expect.equals('Success', initBuffer(""));
+  Expect.equals('Success', initBuffer("string"));
+  Expect.equals('Error', initBuffer([]));
+  Expect.equals('Error', initBuffer([1]));
+  Expect.equals('Error', initBuffer(new Object()));
 }