Type tracing bug fix for issue 28919.

tests/compiler/dart2js/closure_tracer_28919_test.dart

+// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:expect/expect.dart';
+import "package:async_helper/async_helper.dart";
+
+import 'compiler_helper.dart';
+import 'type_mask_test_helper.dart';
+import 'package:compiler/src/types/types.dart';
+
+bool isContainer(TypeMask mask) {
+  return mask is ContainerTypeMask;
+}
+
+const String TEST = '''
+
+foo1() {
+  final methods = [];
+  var res, sum;
+  for (int i = 0; i != 3; i++) {
+    methods.add((int x) { res = x; sum = x + i; });
+  }
+  methods[0](499);
+  probe1res(res);
+  probe1sum(sum);
+  probe1methods(methods);
+}
+probe1res(x) => x;
+probe1sum(x) => x;
+probe1methods(x) => x;
+
+nonContainer(choice) {
+  var m = choice == 0 ? [] : "<String>";
+  if (m is !List) throw 123;
+  // The union then filter leaves us with a non-container type.
+  return m;
+}
+
+foo2(int choice) {
+  final methods = nonContainer(choice);
+  var res, sum;
+  for (int i = 0; i != 3; i++) {
+    methods.add((int x) { res = x; sum = x + i; });
+  }
+  methods[0](499);
+  probe2res(res);
+  probe2methods(methods);
+}
+probe2res(x) => x;
+probe2methods(x) => x;
+
+main() {
+  foo1();
+  foo2(0);
+  foo2(1);
+}
+''';
+
+void main() {
+  Uri uri = new Uri(scheme: 'source');
+  var compiler = compilerFor(TEST, uri);
+  asyncTest(() => compiler.run(uri).then((_) {
+        var typesInferrer = compiler.globalInference.typesInferrerInternal;
+        var closedWorld = typesInferrer.closedWorld;
+        var commonMasks = closedWorld.commonMasks;
+
+        typeOf(String name) {
+          return typesInferrer
+              .getReturnTypeOfElement(findElement(compiler, name));
+        }
+
+        checkType(String name, type) {
+          var mask = typeOf(name);
+          Expect.equals(type.nullable(), simplify(mask, closedWorld), name);
+        }
+
+        checkContainer(String name, bool value) {
+          var mask = typeOf(name);
+          Expect.equals(
+              value, isContainer(mask), '$name is container (mask: $mask)');
+        }
+
+        checkContainer('probe1methods', true);
+        checkType('probe1res', commonMasks.uint31Type);
+        checkType('probe1sum', commonMasks.positiveIntType);
+
+        checkContainer('probe2methods', false);
+        checkType('probe2res', commonMasks.dynamicType);
+      }));
+}