Handle labelled breaks

tests/compiler/dart2js/kernel/compile_from_dill_test_helper.dart

 // Copyright (c) 2017, 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.
 
 // Helper to test compilation equivalence between source and .dill based
 // compilation.
 library dart2js.kernel.compile_from_dill_test_helper;
 
 import 'dart:async';
 
@@ skipping 146 matching lines @@
 
 main() {
   var x;
   int i = 0;
   do {
     if (i == 5) continue;
     x = i;
     if (i == 7) break;
     i++;
   } while (i < 10);
+  outer: for (var a in [3, 5]) {
+    for (var b in [2, 4]) {
+      if (a == b) break outer;
+    }
+  }
   print(x);
 }
 '''
   }, expectIdenticalOutput: false),
   const Test(const {
     'main.dart': '''
 class A<U,V> {
   var a = U;
   var b = V;
 }
@@ skipping 192 matching lines @@
       Map<String, BufferedOutputSink> map2 = collector2.outputMap[outputType];
       checkSets(map1.keys, map2.keys, 'output', equality);
       map1.forEach((String name, BufferedOutputSink output1) {
         BufferedOutputSink output2 = map2[name];
         Expect.stringEquals(output1.text, output2.text);
       });
     });
   }
   return ResultKind.success;
 }