Add basic AllocationToken entity and tests

Logic actually using this entity to follow in subsequent CLs introducing
the command to generate/save these entities as well as the flow logic for
considering them during domain EPP operations.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=181342600

java/google/registry/model/EntityClasses.java

@@ -23,6 +23,7 @@ import google.registry.model.common.Cursor;
 import google.registry.model.common.EntityGroupRoot;
 import google.registry.model.common.GaeUserIdConverter;
 import google.registry.model.contact.ContactResource;
+import google.registry.model.domain.AllocationToken;
 import google.registry.model.domain.DomainApplication;
 import google.registry.model.domain.DomainBase;
 import google.registry.model.domain.DomainResource;
@@ -62,6 +63,7 @@ public final class EntityClasses {
   @SuppressWarnings("unchecked") // varargs
   public static final ImmutableSet<Class<? extends ImmutableObject>> ALL_CLASSES =
       ImmutableSet.of(
+          AllocationToken.class,
           BillingEvent.Cancellation.class,
           BillingEvent.Modification.class,
           BillingEvent.OneTime.class,

java/google/registry/model/domain/AllocationToken.java

@@ -0,0 +1,77 @@
+// Copyright 2017 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package google.registry.model.domain;
+
+import static com.google.common.base.Preconditions.checkArgument;
+import static google.registry.util.PreconditionsUtils.checkArgumentNotNull;
+
+import com.googlecode.objectify.Key;
+import com.googlecode.objectify.annotation.Entity;
+import com.googlecode.objectify.annotation.Id;
+import google.registry.model.BackupGroupRoot;
+import google.registry.model.Buildable;
+import google.registry.model.annotations.ReportedOn;
+import google.registry.model.reporting.HistoryEntry;
+
+/** An entity representing an allocation token. */
+@ReportedOn
+@Entity
+public class AllocationToken extends BackupGroupRoot implements Buildable {
+
+  /** The allocation token string. */
+  @Id String token;
+
+  /** The key of the history entry for which the token was used. Null if not yet used. */
+  Key<HistoryEntry> redemptionHistoryEntry;
+
+  public String getToken() {
+    return token;
+  }
+
+  public Key<HistoryEntry> getRedemptionHistoryEntry() {
+    return redemptionHistoryEntry;
+  }
+
+  public boolean isRedeemed() {
+    return redemptionHistoryEntry != null;
+  }
+
+  @Override
+  public Builder asBuilder() {
+    return new Builder(clone(this));
+  }
+
+  /** A builder for constructing {@link AllocationToken} objects, since they are immutable. */
+  public static class Builder extends Buildable.Builder<AllocationToken> {
+    public Builder() {}
+
+    private Builder(AllocationToken instance) {
+      super(instance);
+    }
+
+    public Builder setToken(String token) {
+      checkArgumentNotNull(token, "token must not be null");
+      checkArgument(!token.isEmpty(), "token must not be blank");
+      getInstance().token = token;
+      return this;
+    }
+
+    public Builder setRedemptionHistoryEntry(Key<HistoryEntry> redemptionHistoryEntry) {
+      getInstance().redemptionHistoryEntry =
+          checkArgumentNotNull(redemptionHistoryEntry, "redemptionHistoryEntry must not be null");
+      return this;
+    }
+  }
+}