First stab at parsing redirecting constructors.

lib/compiler/implementation/tree_validator.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class TreeValidatorTask extends CompilerTask {
   TreeValidatorTask(Compiler compiler) : super(compiler);
 
   void validate(Node tree) {
     assert(check(tree));
   }
@@ skipping 34 matching lines @@
       expect(node, node.assignmentOperator is Operator);
       if (node.isIndex) {
         expect(node.arguments.tail.head, node.arguments.tail.isEmpty());
       } else {
         expect(node.arguments.head, node.arguments.isEmpty());
       }
     } else {
       expect(node, !node.arguments.isEmpty());
     }
   }
+
+  visitReturn(Return node) {
+    if (!node.isRedirectingConstructorBody && node.hasExpression) {
+      // We allow non-expression expressions in Return nodes, but only when
+      // using them to hold redirecting factory constructors.
+      expect(node, node.expression.asExpression() != null);
+    }
+  }
 }
 
 class InvalidNodeError {
   final Node node;
   final String message;
   InvalidNodeError(this.node, [this.message]);
 
   toString() {
     String nodeString = node.toDebugString();
     String result = 'invalid node: $nodeString';
     if (message !== null) result = '$result ($message)';
     return result;
   }
 }