Second round of cleanups for new optional parameter semantics.

lib/compiler/implementation/scanner/string_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.
 
 /**
  * Scanner that reads from a String and creates tokens that points to
  * substrings.
  */
 class StringScanner extends ArrayBasedScanner<SourceString> {
   final String string;
 
-  StringScanner(String this.string, [bool includeComments = false])
+  StringScanner(String this.string, {bool includeComments: false})
     : super(includeComments);
 
   int nextByte() => charAt(++byteOffset);
 
   int peek() => charAt(byteOffset + 1);
 
   int charAt(index)
       => (string.length > index) ? string.charCodeAt(index) : $EOF;
 
   SourceString asciiString(int start, int offset) {
@@ skipping 43 matching lines @@
     assert(0 <= terminal);
     assert(initial + terminal <= internalString.length);
     return new SubstringWrapper(internalString,
                                 begin + initial, end - terminal);
   }
 
   bool isEmpty() => begin == end;
 
   bool isPrivate() => !isEmpty() && identical(internalString.charCodeAt(begin), $_);
 }