Initialize boolean to fix checked mode error.

pkg/kernel/lib/text/ast_to_text.dart

 // Copyright (c) 2016, 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.
 library kernel.ast_to_text;
 
 import '../ast.dart';
 import '../import_table.dart';
 import '../type_propagation/type_propagation.dart';
 
 class Namer<T> {
@@ skipping 173 matching lines @@
   bool showOffsets;
 
   static int SPACE = 0;
   static int WORD = 1;
   static int SYMBOL = 2;
   int state = SPACE;
 
   Printer(this.sink,
       {NameSystem syntheticNames,
       this.showExternal,
-      this.showOffsets,
+      this.showOffsets: false,
       this.importTable,
       this.annotator: const InferredValueAnnotator()})
       : this.syntheticNames = syntheticNames ?? new NameSystem();
 
   Printer._inner(Printer parent, this.importTable)
       : sink = parent.sink,
         syntheticNames = parent.syntheticNames,
         annotator = parent.annotator,
         showExternal = parent.showExternal,
         showOffsets = parent.showOffsets;
@@ skipping 1339 matching lines @@
   }
   throw 'illegal ProcedureKind: $kind';
 }
 
 class ExpressionPrinter {
   final Printer writeer;
   final int minimumPrecedence;
 
   ExpressionPrinter(this.writeer, this.minimumPrecedence);
 }