Merge functionality of HExactType into HBoundedType and fix computation of unions and intersections.

tests/language/switch_this_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.
+
+// Test comparison of this and a constant.
+
+class Foo {
+  static final C1 = const Foo('C1');
+  static final C2 = const Foo('C2');
+  static final C3 = const Foo('C3');
+
+  final name;
+  const Foo(this.name);
+
+  foo() {
+    switch (this) {
+      case C1: return C1;
+      case C2: return C2;
+      default: return null;
+    }
+  }
+}
+
+main() {
+  Expect.equals(Foo.C1, Foo.C1.foo());
+  Expect.equals(Foo.C2, Foo.C2.foo());
+  Expect.equals(null, Foo.C3.foo());
+}