cps_ir: Add JS checks, use argument nullability

pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart

Index: pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
diff --git a/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart b/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
index 63d5f86ad2f1e3ab65decdc65f359bf6a657aff9..bd392df278f2492f3d1da50bd91357a9a6a8b287 100644
--- a/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
+++ b/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
@@ -1144,10 +1144,7 @@ class StatementRewriter extends Transformer implements Pass {
     // 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);