dart2js cps: Add instruction for null checks.

pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart

 // Copyright (c) 2014, 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 tree_ir_nodes;
 
 import '../constants/values.dart' as values;
 import '../dart_types.dart' show DartType, InterfaceType, TypeVariableType;
 import '../elements/elements.dart';
 import '../io/source_information.dart' show SourceInformation;
@@ skipping 939 matching lines @@
 
   accept(StatementVisitor visitor) {
     return visitor.visitYield(this);
   }
 
   accept1(StatementVisitor1 visitor, arg) {
     return visitor.visitYield(this, arg);
   }
 }
 
+class NullCheck extends Statement {
+  Expression condition;
+  Expression value;
+  Selector selector;
+  Statement next;
+  SourceInformation sourceInformation;
+
+  NullCheck({this.condition, this.value, this.selector, this.next,
+      this.sourceInformation});
+
+  accept(StatementVisitor visitor) {
+    return visitor.visitNullCheck(this);
+  }
+
+  accept1(StatementVisitor1 visitor, arg) {
+    return visitor.visitNullCheck(this, arg);
+  }
+}
+
 abstract class ExpressionVisitor<E> {
   E visitExpression(Expression node) => node.accept(this);
   E visitVariableUse(VariableUse node);
   E visitAssign(Assign node);
   E visitInvokeStatic(InvokeStatic node);
   E visitInvokeMethod(InvokeMethod node);
   E visitInvokeMethodDirectly(InvokeMethodDirectly node);
   E visitInvokeConstructor(InvokeConstructor node);
   E visitConstant(Constant node);
   E visitThis(This node);
@@ skipping 70 matching lines @@
   S visitBreak(Break node);
   S visitContinue(Continue node);
   S visitIf(If node);
   S visitWhileTrue(WhileTrue node);
   S visitFor(For node);
   S visitExpressionStatement(ExpressionStatement node);
   S visitTry(Try node);
   S visitUnreachable(Unreachable node);
   S visitForeignStatement(ForeignStatement node);
   S visitYield(Yield node);
+  S visitNullCheck(NullCheck node);
 }
 
 abstract class StatementVisitor1<S, A> {
   S visitStatement(Statement node, A arg) => node.accept1(this, arg);
   S visitLabeledStatement(LabeledStatement node, A arg);
   S visitReturn(Return node, A arg);
   S visitThrow(Throw node, A arg);
   S visitRethrow(Rethrow node, A arg);
   S visitBreak(Break node, A arg);
   S visitContinue(Continue node, A arg);
   S visitIf(If node, A arg);
   S visitWhileTrue(WhileTrue node, A arg);
   S visitFor(For node, A arg);
   S visitExpressionStatement(ExpressionStatement node, A arg);
   S visitTry(Try node, A arg);
   S visitUnreachable(Unreachable node, A arg);
   S visitForeignStatement(ForeignStatement node, A arg);
   S visitYield(Yield node, A arg);
+  S visitNullCheck(NullCheck node, A arg);
 }
 
 abstract class RecursiveVisitor implements StatementVisitor, ExpressionVisitor {
   visitExpression(Expression e) => e.accept(this);
   visitStatement(Statement s) => s.accept(this);
 
   visitVariable(Variable variable) {}
 
   visitVariableUse(VariableUse node) {
     visitVariable(node.variable);
@@ skipping 192 matching lines @@
   }
 
   visitAwait(Await node) {
     visitExpression(node.input);
   }
 
   visitYield(Yield node) {
     visitExpression(node.input);
     visitStatement(node.next);
   }
+
+  visitNullCheck(NullCheck node) {
+    if (node.condition != null) visitExpression(node.condition);
+    visitExpression(node.value);
+    visitStatement(node.next);
+  }
 }
 
 abstract class Transformer implements ExpressionVisitor<Expression>,
                                       StatementVisitor<Statement> {
    Expression visitExpression(Expression e) => e.accept(this);
    Statement visitStatement(Statement s) => s.accept(this);
 }
 
 class RecursiveTransformer extends Transformer {
   void _replaceExpressions(List<Expression> list) {
@@ skipping 237 matching lines @@
   visitAwait(Await node) {
     node.input = visitExpression(node.input);
     return node;
   }
 
   visitYield(Yield node) {
     node.input = visitExpression(node.input);
     node.next = visitStatement(node.next);
     return node;
   }
+
+  visitNullCheck(NullCheck node) {
+    if (node.condition != null) {
+      node.condition = visitExpression(node.condition);
+    }
+    node.value = visitExpression(node.value);
+    node.next = visitStatement(node.next);
+    return node;
+  }
 }
 
 class FallthroughTarget {
   final Statement target;
   int useCount = 0;
 
   FallthroughTarget(this.target);
 }
 
 /// A stack machine for tracking fallthrough while traversing the Tree IR.
@@ skipping 17 matching lines @@
 
   /// Number of uses of the current fallthrough target.
   int get useCount => _stack.last.useCount;
 
   /// Indicate that a statement will fall through to the current fallthrough
   /// target.
   void use() {
     ++_stack.last.useCount;
   }
 }