Added TODO.

lib/compiler/implementation/tree/dartstring.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.
 
 /**
  * The [DartString] type represents a Dart string value as a sequence of Unicode
  * Scalar Values.
  * After parsing, any valid [LiteralString] will contain a [DartString]
  * representing its content after removing quotes and resolving escapes in
  * its source.
  */
-class DartString implements Iterable<int> {
+abstract class DartString implements Iterable<int> {
   factory DartString.empty() => const LiteralDartString("");
   // This is a convenience constructor. If you need a const literal DartString,
   // use [const LiteralDartString(string)] directly.
   factory DartString.literal(String string) => new LiteralDartString(string);
   factory DartString.rawString(SourceString source, int length) =>
       new RawSourceDartString(source, length);
   factory DartString.escapedString(SourceString source, int length) =>
       new EscapedSourceDartString(source, length);
   factory DartString.concat(DartString first, DartString second) {
     if (first.isEmpty()) return second;
@@ skipping 30 matching lines @@
   const LiteralDartString(this.string);
   int get length => string.length;
   Iterator<int> iterator() => new StringCodeIterator(string);
   String slowToString() => string;
   SourceString get source => new StringWrapper(string);
 }
 
 /**
  * A [DartString] where the content comes from a slice of the program source.
  */
-class SourceBasedDartString extends DartString {
+abstract class SourceBasedDartString extends DartString {
   String toStringCache = null;
   final SourceString source;
   final int length;
   SourceBasedDartString(this.source, this.length);
   abstract Iterator<int> iterator();
 }
 
 /**
  * Special case of a [SourceBasedDartString] where we know the source doesn't
  * contain any escapes.
@@ skipping 126 matching lines @@
       for (int i = 0; i < 3; i++) {
         code = source.next();
         value = value * 16 + hexDigitValue(code);
       }
       return value;
     }
     return code;
   }
 }