Create class AST nodes.

dart/frog/world.dart

Index: dart/frog/world.dart
diff --git a/dart/frog/world.dart b/dart/frog/world.dart
index 92e90250e6bb4346dc5d3b6138ffd1e8fad85f54..2110844e75d66ed7cad3fa2a56ed3bb7eade4745 100644
--- a/dart/frog/world.dart
+++ b/dart/frog/world.dart
@@ -262,11 +262,13 @@ class World {
     return !hasErrors;
   }
 
+  /** Returns true if Leg handled the compilation job. */
   bool runLeg() {
     if (!options.enableLeg) return false;
     bool res = withTiming('try leg compile', () => leg.compile(this));
     if (!res && options.legOnly) {
       fatal("Leg could not compile ${options.dartScript}");
+      return true; // In --leg_only, always "handle" the compilation.
     }
     return res;
   }