Fix bug in container tracer, where a local would be updated with the wrong type in the expression `…

tests/compiler/dart2js/list_tracer3_test.dart

Index: tests/compiler/dart2js/list_tracer3_test.dart
===================================================================
--- tests/compiler/dart2js/list_tracer3_test.dart	(revision 27876)
+++ tests/compiler/dart2js/list_tracer3_test.dart	(working copy)
@@ -16,10 +16,12 @@
 
 
 const String TEST = r'''
-var myList = [42];
+var myList = [];
+var otherList = ['foo', 42];
 main() {
-  var a = new List(42);
-  return myList[0];
+  var a = otherList[0];
+  a += 54;
+  myList.add(a);
 }
 ''';
 
@@ -35,6 +37,9 @@
       Expect.equals(type, mask.elementType.simplify(compiler), name);
     }
 
-    checkType('myList', compiler.typesTask.intType);
+    var interceptorType =
+      findTypeMask(compiler, 'Interceptor', 'nonNullSubclass');
+
+    checkType('myList', interceptorType);
   }));
 }