Fix warnings.

lib/compiler/implementation/js/nodes.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.
 
 interface NodeVisitor<T> {
   T visitProgram(Program node);
 
   T visitBlock(Block node);
   T visitExpressionStatement(ExpressionStatement node);
   T visitEmptyStatement(EmptyStatement node);
@@ skipping 176 matching lines @@
   accept(NodeVisitor visitor) => visitor.visitEmptyStatement(this);
   void visitChildren(NodeVisitor visitor) {}
 }
 
 class If extends Statement {
   final Expression condition;
   final Node then;
   final Node otherwise;
 
   If(this.condition, this.then, this.otherwise);
-  If.then(this.condition, this.then) : this.otherwise = new EmptyStatement();
+  If.noElse(this.condition, this.then) : this.otherwise = new EmptyStatement();
 
   bool get hasElse => otherwise is !EmptyStatement;
 
   accept(NodeVisitor visitor) => visitor.visitIf(this);
 
   void visitChildren(NodeVisitor visitor) {
     condition.accept(visitor);
     then.accept(visitor);
     otherwise.accept(visitor);
   }
@@ skipping 622 matching lines @@
   /** Contains the pattern and the flags.*/
   String pattern;
 
   RegExpLiteral(this.pattern);
 
   accept(NodeVisitor visitor) => visitor.visitRegExpLiteral(this);
   void visitChildren(NodeVisitor visitor) {}
 
   int get precedenceLevel => PRIMARY;
 }