First step towards having patch files for generic libraries.

lib/compiler/implementation/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('colors.dart', prefix: 'colors');
 
 /**
  * Represents a file of source code.
@@ skipping 24 matching lines @@
     }
     return _lineStarts;
   }
 
   int getLine(int position) {
     // TODO(jimhug): Implement as binary search.
     var starts = lineStarts;
     for (int i=0; i < starts.length; i++) {
       if (starts[i] > position) return i-1;
     }
-    throw 'bad position';
+    throw 'bad position #$position in file $filename with '
+          'length ${text.length}.';
   }
 
   int getColumn(int line, int position) {
     return position - lineStarts[line];
   }
 
   /**
    * Create a pretty string representation from a character position
    * in the file.
    */
@@ skipping 25 matching lines @@
       }
 
       for (; i < toColumn; i++) {
         buf.add(color('^'));
       }
     }
 
     return buf.toString();
   }
 }