Add optional message to assert in Dart2js.

pkg/compiler/lib/src/tree/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.
 
 part of tree;
 
 abstract class Visitor<R> {
   const Visitor();
 
   R visitNode(Node node);
 
+  R visitAssert(Assert node) => visitStatement(node);
   R visitAsyncForIn(AsyncForIn node) => visitLoop(node);
   R visitAsyncModifier(AsyncModifier node) => visitNode(node);
   R visitAwait(Await node) => visitExpression(node);
   R visitBlock(Block node) => visitStatement(node);
   R visitBreakStatement(BreakStatement node) => visitGotoStatement(node);
   R visitCascade(Cascade node) => visitExpression(node);
   R visitCascadeReceiver(CascadeReceiver node) => visitExpression(node);
   R visitCaseMatch(CaseMatch node) => visitNode(node);
   R visitCatchBlock(CatchBlock node) => visitNode(node);
   R visitClassNode(ClassNode node) => visitNode(node);
@@ skipping 117 matching lines @@
   toDebugString() {
     return PrettyPrinter.prettyPrint(this);
   }
 
   String getObjectDescription() => super.toString();
 
   Token getBeginToken();
 
   Token getEndToken();
 
+  Assert asAssert() => null;
   AsyncModifier asAsyncModifier() => null;
   Await asAwait() => null;
   Block asBlock() => null;
   BreakStatement asBreakStatement() => null;
   Cascade asCascade() => null;
   CascadeReceiver asCascadeReceiver() => null;
   CaseMatch asCaseMatch() => null;
   CatchBlock asCatchBlock() => null;
   ClassNode asClassNode() => null;
   Combinator asCombinator() => null;
@@ skipping 1041 matching lines @@
 
   visitChildren(Visitor visitor) {
     expression.accept(visitor);
   }
 
   Token getBeginToken() => awaitToken;
 
   Token getEndToken() => expression.getEndToken();
 }
 
+class Assert extends Statement {
+  final Token assertToken;
+  final Expression condition;
+  /** Message may be `null`. */
+  final Expression message;
+  final Token semicolonToken;
+
+  Assert(this.assertToken, this.condition, this.message, this.semicolonToken);
+
+  Assert asAssert() => this;
+
+  bool get hasMessage => message != null;
+
+  accept(Visitor visitor) => visitor.visitAssert(this);
+
+  visitChildren(Visitor visitor) {
+    condition.accept(visitor);
+    if (message != null) message.accept(visitor);
+  }
+
+  Token getBeginToken() => assertToken;
+  Token getEndToken() => semicolonToken;
+}
+
 class Rethrow extends Statement {
   final Token throwToken;
   final Token endToken;
 
   Rethrow(this.throwToken, this.endToken);
 
   Rethrow asRethrow() => this;
 
   accept(Visitor visitor) => visitor.visitRethrow(this);
   visitChildren(Visitor visitor) { }
@@ skipping 1123 matching lines @@
 
   // VariableDefinitions.
   get metadata => null;
   get type => null;
 
   // Typedef.
   get typeParameters => null;
   get formals => null;
   get typedefKeyword => null;
 }