Fix dart2js warnings and add test to ensure it stays clean.

pkg/compiler/lib/src/io/line_column_provider.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.
 
 library dart2js.io.line_column;
 
-import 'code_output.dart';
+import 'code_output.dart' show CodeOutputListener;
 
 /// Interface for providing line/column information.
 abstract class LineColumnProvider {
   /// Returns the line number (0-based) for [offset].
   int getLine(int offset);
 
   /// Returns the column number (0-based) for [offset] at the given [line].
   int getColumn(int line, int offset);
 
   /// Returns the offset for 0-based [line] and [column] numbers.
@@ skipping 55 matching lines @@
   @override
   void onDone(int length) {
     lineStarts.add(length + 1);
     this.length = length;
   }
 
   String toString() {
     return 'lineStarts=$lineStarts,length=$length';
   }
 }