Added TODO.

lib/compiler/implementation/tree/dartstring.dart

Index: lib/compiler/implementation/tree/dartstring.dart
diff --git a/lib/compiler/implementation/tree/dartstring.dart b/lib/compiler/implementation/tree/dartstring.dart
index d6b05895fecd6f8db02c0998d20b9dda044eca8e..30198fabcfd3b74dfadb1c18e9f5d4da11b66093 100644
--- a/lib/compiler/implementation/tree/dartstring.dart
+++ b/lib/compiler/implementation/tree/dartstring.dart
@@ -9,7 +9,7 @@
  * 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.
@@ -60,7 +60,7 @@ class LiteralDartString extends DartString {
 /**
  * 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;