Make StringBuffer and interpolation behave the same.

dart/sdk/lib/core/string_buffer.dart

 // Copyright (c) 2011, 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.
 
 part of dart.core;
 
 /**
  * The StringBuffer class is useful for concatenating strings
  * efficiently. Only on a call to [toString] are the strings
  * concatenated to a single String.
@@ skipping 36 matching lines @@
     add(content);
   }
 
   /// Returns the length of the buffer.
   int get length => _length;
 
   bool get isEmpty => _length == 0;
 
   /// Adds [obj] to the buffer.
   void add(Object obj) {
+    // TODO(srdjan): The following four lines could be replaced by
+    // '$obj', but apparently this is too slow on the Dart VM.
     String str = obj.toString();
-    if (str == null || str.isEmpty) return;
+    if (str is !String) {
+      throw new ArgumentError('toString() did not return a string');
+    }
+    if (str.isEmpty) return;
     _buffer.add(str);
     _length += str.length;
   }
 
   /// Adds all items in [objects] to the buffer.
   void addAll(Collection objects) {
     for (Object obj in objects) add(obj);
   }
 
   /// Adds the string representation of [charCode] to the buffer.
@@ skipping 12 matching lines @@
     if (_buffer.length == 0) return "";
     if (_buffer.length == 1) return _buffer[0];
     String result = Strings.concatAll(_buffer);
     _buffer.clear();
     _buffer.add(result);
     // Since we track the length at each add operation, there is no
     // need to update it in this function.
     return result;
   }
 }