Add compiler error when instantiating abstract class

tests/language/interface_negative_test.dart

+// Copyright (c) 2012, 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.
+// Dart test program for testing Interfaces.
+
+interface Ai {
+  int foo();
+}
+
+interface Bi extends Ai default InterfaceTest {
+  Bi();
+}
+
+interface Simple extends Ai { }
+
+interface Aai { }
+
+interface Abi { }
+
+interface Bar { }
+
+interface Foo extends Bar { }
+
+interface Baz extends Bar, Foo { }
+
+class InterfaceTest implements Ai, Aai, Abi, Baz, Bi {
+  var f;
+
+  InterfaceTest() {}
+  int foo() { return 1; }
+
+  // intentionally unimplemented methods
+  abstract beta();
+  abstract String beta1();
+  abstract String beta2(double d);
+}
+
+main() {
+  // instantiate an abstract class
+  var o = new Bi(); /// compile-time error
+  Expect.equals(1, o.foo());
+}