Remove all warnings from dart2js and add a test to prevent introduction of new.

dart/sdk/lib/_internal/compiler/implementation/scanner/array_based_scanner.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 scanner_implementation;
 
 abstract
 class ArrayBasedScanner<S extends SourceString> extends AbstractScanner<S> {
   int get charOffset => byteOffset + extraCharOffset;
   final Token tokens;
@@ skipping 161 matching lines @@
     appendByteStringToken(COMMENT_INFO, value);
   }
 
   void discardOpenLt() {
     while (!groupingStack.isEmpty
         && identical(groupingStack.head.kind, LT_TOKEN)) {
       groupingStack = groupingStack.tail;
     }
   }
 
-  // TODO(ahe): make class abstract instead of adding an abstract method.
-  peek();
+  void unmatchedBeginGroup(BeginGroupToken begin);
 }