Create a new scope when analyzing then and else of an if.

sdk/lib/_internal/compiler/implementation/resolution/members.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.
 
 part of resolution;
 
 abstract class TreeElements {
   Element get currentElement;
   Set<Node> get superUses;
 
@@ skipping 1935 matching lines @@
     statementScope = oldStatementScope;
 
     scope = oldScope;
     enclosingElement = previousEnclosingElement;
 
     world.registerInstantiatedClass(compiler.functionClass, mapping);
   }
 
   visitIf(If node) {
     visit(node.condition);
-    visit(node.thenPart);
-    visit(node.elsePart);
+    visitIn(node.thenPart, new BlockScope(scope));
+    visitIn(node.elsePart, new BlockScope(scope));
   }
 
   static bool isLogicalOperator(Identifier op) {
     String str = op.source.stringValue;
     return (identical(str, '&&') || str == '||' || str == '!');
   }
 
   Element resolveSend(Send node) {
     Selector selector = resolveSelector(node, null);
     if (node.isSuperCall) mapping.superUses.add(node);
@@ skipping 1970 matching lines @@
     return e;
   }
 
   /// Assumed to be called by [resolveRedirectingFactory].
   Element visitReturn(Return node) {
     Node expression = node.expression;
     return finishConstructorReference(visit(expression),
                                       expression, expression);
   }
 }