Reapply "Refactor constant part." (r4958) with fixes.

frog/leg/tree/nodes.dart

Index: frog/leg/tree/nodes.dart
diff --git a/frog/leg/tree/nodes.dart b/frog/leg/tree/nodes.dart
index 1f1fe841506bf644ec7c9895ff83af180008a074..0b63bbbe3c460b4937584ce96ee555923d7bfd5e 100644
--- a/frog/leg/tree/nodes.dart
+++ b/frog/leg/tree/nodes.dart
@@ -729,8 +729,6 @@ class DartString implements Iterable<int> {
   DartString();
   abstract int get length();
   bool isEmpty() => length == 0;
-  // TODO(lrn): Is this test too expensive?
-  bool definitelyEquals(DartString other) => toString() == other.toString();
   abstract Iterator<int> iterator();
   abstract String toString();