Fix result type checking of factory constructors of generic classes in VM.

runtime/lib/string_base.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.
 
 /**
  * [_StringBase] contains common methods used by concrete String
  * implementations, e.g., _OneByteString.
  */
 class _StringBase {
 
@@ skipping 406 matching lines @@
   // whitespaces for one byte strings.
   bool _isWhitespace(int codePoint) {
     return
       (codePoint === 32) || // Space.
       ((9 <= codePoint) && (codePoint <= 13)); // CR, LF, TAB, etc.
   }
 }
 
 
 class _ExternalTwoByteString extends _StringBase implements String {
-  factory ExternalTwoByteString._uninstantiable() {
+  factory _ExternalTwoByteString._uninstantiable() {
     throw new UnsupportedError(
         "_ExternalTwoByteString can only be allocated by the VM");
   }
 
   // Checks for one-byte whitespaces only.
   // TODO(srdjan): Investigate if 0x85 (NEL) and 0xA0 (NBSP) are valid
   // whitespaces. Add checking for multi-byte whitespace codepoints.
   bool _isWhitespace(int codePoint) {
     return
       (codePoint === 32) || // Space.
@@ skipping 40 matching lines @@
     for (int g in groups) {
       result.add(group(g));
     }
     return result;
   }
 
   final int start;
   final String str;
   final String pattern;
 }