dart2js cps: Fix a couple of minor issues.

pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart

 // Copyright (c) 2014, 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.
 
 library tree_ir_builder;
 
 import '../dart2jslib.dart' as dart2js;
 import '../elements/elements.dart';
 import '../cps_ir/cps_ir_nodes.dart' as cps_ir;
 import '../util/util.dart' show CURRENT_ELEMENT_SPANNABLE;
@@ skipping 302 matching lines @@
     if (variable == null) return visit(node.body);
 
     Expression value = visit(node.primitive);
     return Assign.makeStatement(variable, value, visit(node.body));
   }
 
   Statement visitLetCont(cps_ir.LetCont node) {
     // Introduce labels for continuations that need them.
     int safeForInliningLengthOnEntry = safeForInlining.length;
     for (cps_ir.Continuation continuation in node.continuations) {
-      if (continuation.hasMultipleUses) {
+      if (continuation.hasMultipleUses || continuation.isRecursive) {

[asgerf, 2015/06/29 14:09:12]

If the continuation is marked as recursive, but all of its recursive invocations
have been eliminated, we still need the label because we translate it into a
while. This is sort of bad, but it's no reason to crash the compiler.

[Kevin Millikin (Google), 2015/06/29 14:43:28]

The way we do it now has always seemed brittle; we should have something better.
 But I agree we should not crash and this is a simple change to avoid that.

         labels[continuation] = new Label();
       } else {
         safeForInlining.add(continuation);
       }
     }
     Statement body = visit(node.body);
     safeForInlining.length = safeForInliningLengthOnEntry;
     // Continuations are bound at the same level, but they have to be
     // translated as if nested.  This is because the body can invoke any
     // of them from anywhere, so it must be nested inside all of them.
@@ skipping 289 matching lines @@
       return new ForeignStatement(
           node.codeTemplate,
           node.type,
           node.arguments.map(getVariableUse).toList(growable: false),
           node.nativeBehavior,
           node.dependency);
     }
   }
 }