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

frog/tests/leg/src/ConstantFoldingTest.dart

Index: frog/tests/leg/src/ConstantFoldingTest.dart
diff --git a/frog/tests/leg/src/ConstantFoldingTest.dart b/frog/tests/leg/src/ConstantFoldingTest.dart
index aabc92c51e0d194c4b1ecf88eb607666f71e096d..b8faaf35bcf27df493f2dc41ffbf1b9091274556 100644
--- a/frog/tests/leg/src/ConstantFoldingTest.dart
+++ b/frog/tests/leg/src/ConstantFoldingTest.dart
@@ -38,9 +38,9 @@ void compileAndTest(String code, String entry, RegExp regexp) {
 
 main() {
   compileAndTest(
-      NUMBER_FOLDING, 'main', const RegExp(@"print\$1\(7\)"));
+      NUMBER_FOLDING, 'main', const RegExp(@"print\$1\(\(7\)\)"));
   compileAndTest(
-      NEGATIVE_NUMBER_FOLDING, 'main', const RegExp(@"print\$1\(1\)"));
+      NEGATIVE_NUMBER_FOLDING, 'main', const RegExp(@"print\$1\(\(1\)\)"));
 
   String generated = compile(NULL_EQUALS_FOLDING, 'foo');
   RegExp regexp = const RegExp(@'eqNull\$1\(a\)');
@@ -49,7 +49,7 @@ main() {
   regexp = const RegExp(@'\(?void 0\)? === b');
   Expect.isTrue(regexp.hasMatch(generated));
 
-  regexp = const RegExp('4 === c');
+  regexp = const RegExp(@'\(4\) === c');
   Expect.isTrue(regexp.hasMatch(generated));
 
   regexp = const RegExp("'foo' === d");