VM library tweaks to avoid megamorphic calls in dart2js

runtime/lib/string_patch.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.
 
 const int _maxAscii = 0x7f;
 const int _maxLatin1 = 0xff;
 const int _maxUtf16 = 0xffff;
 const int _maxUnicode = 0x10ffff;
 
 patch class String {
@@ skipping 783 matching lines @@
       buffer.write(onNonMatch(this.substring(startIndex, match.start)));
       buffer.write(onMatch(match).toString());
       startIndex = match.end;
     }
     buffer.write(onNonMatch(this.substring(startIndex)));
     return buffer.toString();
   }
 
   // Convert single object to string.
   static String _interpolateSingle(Object o) {
+    if (o is String) return o;
     final s = o.toString();
     if (s is! String) {
       throw new ArgumentError(s);
     }
     return s;
   }
 
   /**
    * Convert all objects in [values] to strings and concat them
    * into a result string.
@@ skipping 536 matching lines @@
     int end = index + _pattern.length;
     _current = new _StringMatch(index, _input, _pattern);
     // Empty match, don't start at same location again.
     if (end == _index) end++;
     _index = end;
     return true;
   }
 
   Match get current => _current;
 }