Implement correct scoping rules for variables.

sdk/lib/_internal/compiler/implementation/closure.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 closureToClassMapper;
 
 import "elements/elements.dart";
 import "dart2jslib.dart";
 import "dart_types.dart";
 import "scanner/scannerlib.dart" show Token;
@@ skipping 447 matching lines @@
 
   visitVariableDefinitions(VariableDefinitions node) {
     if (node.type != null) {
       visit(node.type);
     }
     for (Link<Node> link = node.definitions.nodes;
          !link.isEmpty;
          link = link.tail) {
       Node definition = link.head;
       Element element = elements[definition];
-      assert(element != null);
+      assert(invariant(definition, element != null,
+          message: "unresolved local definition"));
       declareLocal(element);
       // We still need to visit the right-hand sides of the init-assignments.
       // For SendSets don't visit the left again. Otherwise it would be marked
       // as mutated.
       if (definition is Send) {
         Send assignment = definition;
         Node arguments = assignment.argumentsNode;
         if (arguments != null) {
           visit(arguments);
         }
@@ skipping 358 matching lines @@
   }
 
   visitTryStatement(TryStatement node) {
     // TODO(ngeoffray): implement finer grain state.
     bool oldInTryStatement = inTryStatement;
     inTryStatement = true;
     node.visitChildren(this);
     inTryStatement = oldInTryStatement;
   }
 }