dart2js cps: Emit unlabeled breaks and continues when possible.

tests/compiler/dart2js/js_backend_cps_ir_control_flow_test.dart

Index: tests/compiler/dart2js/js_backend_cps_ir_control_flow_test.dart
diff --git a/tests/compiler/dart2js/js_backend_cps_ir_control_flow_test.dart b/tests/compiler/dart2js/js_backend_cps_ir_control_flow_test.dart
index b16a1a88966c779b110d0bb770a52b7401162a6e..adf8db795e4c5e1608161a4f0f19bb9a454efaf2 100644
--- a/tests/compiler/dart2js/js_backend_cps_ir_control_flow_test.dart
+++ b/tests/compiler/dart2js/js_backend_cps_ir_control_flow_test.dart
@@ -57,13 +57,12 @@ main() {
 }""", """
 function() {
   var i = 0;
-  L2:
-    while (P.identical(V.foo(true), true)) {
-      P.print(1);
-      if (P.identical(V.foo(false), true))
-        break L2;
-      i = V.foo(i);
-    }
+  while (P.identical(V.foo(true), true)) {
+    P.print(1);
+    if (P.identical(V.foo(false), true))
+      break;
+    i = V.foo(i);
+  }
   P.print(2);
 }"""),
 const TestEntry("""