Detect dead code, and handle breaks and continues in inferrer.

sdk/lib/_internal/compiler/implementation/elements/elements.dart

 // Copyright (c) 2012, 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 elements;
 
 import 'dart:uri';
 
 import 'modelx.dart';
 import '../tree/tree.dart';
@@ skipping 530 matching lines @@
     }
     return false;
   }
 
   static bool switchStatementHasDefault(SwitchStatement node) {
     for (SwitchCase switchCase in node.cases) {
       if (switchCase.isDefaultCase) return true;
     }
     return false;
   }
+
+  static bool isUnusedLabel(LabeledStatement node, TreeElements elements) {
+    Node body = node.statement;
+    TargetElement element = elements[body];
+    // Labeled statements with no element on the body have no breaks.
+    // A different target statement only happens if the body is itself
+    // a break or continue for a different target. In that case, this
+    // label is also always unused.
+    return element == null || element.statement != body;
+  }
 }
 
 abstract class ErroneousElement extends Element implements FunctionElement {
   MessageKind get messageKind;
   Map get messageArguments;
 }
 
 abstract class AmbiguousElement extends Element {
   MessageKind get messageKind;
   Map get messageArguments;
@@ skipping 336 matching lines @@
   int get resolutionState;
   Token get beginToken;
   Token get endToken;
 
   // TODO(kasperl): Try to get rid of these.
   void set annotatedElement(Element value);
   void set resolutionState(int value);
 
   MetadataAnnotation ensureResolved(Compiler compiler);
 }