cps_ir: Add JS checks, use argument nullability

pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.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.optimization.statement_rewriter;
 
 import 'optimization.dart' show Pass;
 import '../tree_ir_nodes.dart';
 import '../../io/source_information.dart';
 import '../../elements/elements.dart';
@@ skipping 1126 matching lines @@
   }
 
   Statement getBranch(If node, bool polarity) {
     return polarity ? node.thenStatement : node.elseStatement;
   }
 
   void handleForeignCode(ForeignCode node) {
     // Some arguments will get inserted in a JS code template.  The arguments
     // will not always be evaluated (e.g. the second placeholder in the template
     // '# && #').
-
-    // TODO(sra): Find out which tree_ir expressions are not nullable. It helps
-    // a lot with templates like '#.push(#)'.
-    bool isNullable(e) => true;
+    bool isNullable(int position) => node.nullableArguments[position];
 
     int safeArguments =
       PlaceholderSafetyAnalysis.analyze(node.codeTemplate.ast, isNullable);
     inEmptyEnvironment(() {
       for (int i = node.arguments.length - 1; i >= safeArguments; --i) {
         node.arguments[i] = visitExpression(node.arguments[i]);
       }
     });
     for (int i = safeArguments - 1; i >= 0; --i) {
       node.arguments[i] = visitExpression(node.arguments[i]);
@@ skipping 127 matching lines @@
   VariableUseCallback callback;
 
   VariableUseVisitor(this.callback);
 
   visitVariableUse(VariableUse use) => callback(use);
 
   static void visit(Expression node, VariableUseCallback callback) {
     new VariableUseVisitor(callback).visitExpression(node);
   }
 }