Migrate test block 113 to Dart 2.0.

tests/language_2/generic_field_mixin6_test.dart

+// Copyright (c) 2014, 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.
+
+// Test that generic types in mixins are handled.
+
+import 'package:expect/expect.dart';
+
+class M<T> {
+  T field = 0; //# 01: compile-time error
+}
+
+class A<U> {}
+
+class C1<V> = Object with M<V>;
+class C2 = Object with M<int>;
+class C3 = Object with M<String>;
+
+main() {
+  checkNoDynamicTypeError(() => new C1<int>());

[Jennifer Messerly, 2017/08/25 21:59:48]

i'm not sure this is testing anything now, since the "T field" line will not be
part of the test (in "none" multittest). So all we're doing is executing a bunch
of constructors with no fields.

Here's a suggested change:

class M<T> {
  T field1 = 0; //# 01: compile-time error
  T field2 = 0 as dynamic;
}

main() {
  // no error: 0 is an int
  new C1<int>();
  new C2();

  // type error: 0 is not a string
  Expect.throws(() {
    new C1<String>();
  }, (e) => e is TypeError);
  Expect.throws(() {
    new C3();
  }, (e) => e is TypeError);
}

... this now validates the runtime behavior, that the implementation performs
the correct type check `is T` on the value of 0 at runtime

[jcollins, 2017/08/29 15:41:06]

agreed, this is better.  Done.

+  checkNoDynamicTypeError(() => new C1<String>());
+  checkNoDynamicTypeError(() => new C2());
+  checkNoDynamicTypeError(() => new C3());
+}
+
+/// Checks that no dynamic type error is thrown when [f] is executed regardless
+/// of execution mode.
+void checkNoDynamicTypeError(f()) {
+  try {
+    f();
+  } on TypeError {
+    Expect.fail('Unexpected type error');
+  }
+}