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

frog/tests/leg/src/GVNTest.dart

Index: frog/tests/leg/src/GVNTest.dart
diff --git a/frog/tests/leg/src/GVNTest.dart b/frog/tests/leg/src/GVNTest.dart
index 3dcb1b977de063a5d01d64e5650ebb1d120f645c..5212658c017b4082a05bcb4f0b33d85925c64268 100644
--- a/frog/tests/leg/src/GVNTest.dart
+++ b/frog/tests/leg/src/GVNTest.dart
@@ -13,7 +13,7 @@ void foo(bar) {
 
 main() {
   String generated = compile(TEST_ONE, 'foo');
-  RegExp regexp = const RegExp("1 \\+ bar");
+  RegExp regexp = const RegExp(@"\(1\) \+ bar");
   Iterator matches = regexp.allMatches(generated).iterator();
   Expect.isTrue(matches.hasNext());
   matches.next();