Implement cycle-checking and code generation for redirecting constructors.

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 0219c8fd1398de1c4daf987e0729c954d23bd110..e426490b3762d9f95b88b6d80164dc3c671002e0 100644
--- a/frog/leg/tree/nodes.dart
+++ b/frog/leg/tree/nodes.dart
@@ -531,6 +531,13 @@ class FunctionExpression extends Expression {
     if (returnType !== null) returnType.accept(visitor);
   }
 
+  bool hasBody() {
+    if (body.asReturn() !== null) return true;
+    NodeList statements = body.asBlock().statements;
+    return (!statements.nodes.isEmpty() ||
+            statements.getBeginToken().kind !== $SEMICOLON);
+  }
+
   Token getBeginToken() {
     Token token = firstBeginToken(modifiers, returnType);
     if (token !== null) return token;