Start using the new union/intersection code for HType.INDEXABLE_PRIMITIVE.

sdk/lib/_internal/compiler/implementation/js_backend/backend.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
index 2c966c73f880dc911b552ce8668d70d4c76496d0..61d2ed306730a8fc2ec61480191850a400d1ef31 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
@@ -642,6 +642,7 @@ class JavaScriptBackend extends Backend {
   ClassElement jsFunctionClass;
   ClassElement jsNullClass;
   ClassElement jsBoolClass;
+  ClassElement jsIndexableClass;
   Element jsArrayLength;
   Element jsStringLength;
   Element jsArrayRemoveLast;
@@ -819,6 +820,9 @@ class JavaScriptBackend extends Backend {
           compiler.findInterceptor(const SourceString('JSFunction')),
       jsBoolClass = compiler.findInterceptor(const SourceString('JSBool'))];
 
+    jsIndexableClass =
+        compiler.findInterceptor(const SourceString('JSIndexable'));
+
     jsArrayClass.ensureResolved(compiler);
     jsArrayLength = compiler.lookupElementIn(
         jsArrayClass, const SourceString('length'));