Revert "dart2js: switching isolate is not 'PURE'"

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

Index: pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
index c4ce556be84ffa6eaae42a0073f37fe993a4cdc9..4bdc89fb78775e0f8b79477119de6212a2f52929 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
@@ -2900,7 +2900,7 @@ class IrBuilderVisitor extends ast.Visitor<ir.Primitive>
         return irBuilder.buildForeignCode(
             js.js.parseForeignJS(backend.namer.staticStateHolder),
             const <ir.Primitive>[],
-            NativeBehavior.DEPENDS_OTHER);
+            NativeBehavior.PURE);
 
       case 'JS_SET_STATIC_STATE':
         validateArgumentCount(exactly: 1);
@@ -2910,7 +2910,7 @@ class IrBuilderVisitor extends ast.Visitor<ir.Primitive>
         return irBuilder.buildForeignCode(
             js.js.parseForeignJS("$isolateName = #"),
             <ir.Primitive>[value],
-            NativeBehavior.CHANGES_OTHER);
+            NativeBehavior.PURE);
 
       case 'JS_CALL_IN_ISOLATE':
         validateArgumentCount(exactly: 2);