dart2js cps: Fix a couple of minor issues.

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

Index: pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
diff --git a/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart b/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
index 0d663c551beec084407fc5ed1574fca789ba1e25..6e39aae97a7524a880391cedc57a60d0841a2051 100644
--- a/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
+++ b/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
@@ -320,7 +320,7 @@ class Builder implements cps_ir.Visitor<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);