Add CodeOutput to prepare for emission without in-memory buffers.

pkg/compiler/lib/src/source_file.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 source_file;
 
 import 'dart:math';
 import 'dart:convert' show UTF8;
 
+/// 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);
+}
+
 /**
  * Represents a file of source code. The content can be either a [String] or
  * a UTF-8 encoded [List<int>] of bytes.
  */
-abstract class SourceFile {
+abstract class SourceFile implements LineColumnProvider {
 
   /** The name of the file. */
   String get filename;
 
   /** The text content of the file represented as a String. */
   String slowText();
 
   /** The content of the file represented as a UTF-8 encoded [List<int>]. */
   List<int> slowUtf8Bytes();
 
@@ skipping 188 matching lines @@
 
   int get length => text.length;
   set length(int v) { }
 
   String slowText() => text;
 
   List<int> slowUtf8Bytes() => UTF8.encode(text);
 
   String slowSubstring(int start, int end) => text.substring(start, end);
 }