dart2js cps: More precise rewrite for indexables.

pkg/compiler/lib/src/cps_ir/type_propagation.dart

Index: pkg/compiler/lib/src/cps_ir/type_propagation.dart
diff --git a/pkg/compiler/lib/src/cps_ir/type_propagation.dart b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
index 7fd2153184a842c4c8fcec1039c8b22f15ccc75e..4748a59621bc931528b63d1b7e8d45ce5417427c 100644
--- a/pkg/compiler/lib/src/cps_ir/type_propagation.dart
+++ b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
@@ -1238,8 +1238,8 @@ class TransformingVisitor extends DeepRecursiveVisitor {
         return true;
 
       case '[]=':
-        if (receiverValue.isNullable) return false;
-        if (!typeSystem.isDefinitelyMutableIndexable(receiverValue.type)) {
+        if (!typeSystem.isDefinitelyMutableIndexable(receiverValue.type,
+                allowNull: true)) {
           return false;
         }
         Primitive index = getDartArgument(node, 0);