Add dagger map for injecting DnsWriter implementations

This is one of several CLs in a sequence for allowing per-TLD DNS
implementations.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=129445641

java/google/registry/dns/DnsWriterProxy.java

@@ -0,0 +1,42 @@
+// Copyright 2016 The Domain Registry 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.dns;
+
+import static com.google.common.base.Preconditions.checkState;
+
+import com.google.common.collect.ImmutableMap;
+import google.registry.dns.writer.DnsWriter;
+import google.registry.model.registry.Registry;
+import java.util.Map;
+import javax.inject.Inject;
+
+/** Proxy for retrieving {@link DnsWriter} implementations. */
+public final class DnsWriterProxy {
+
+  private final ImmutableMap<String, DnsWriter> dnsWriters;
+
+  @Inject
+  DnsWriterProxy(Map<String, DnsWriter> dnsWriters) {
+    this.dnsWriters = ImmutableMap.copyOf(dnsWriters);
+  }
+
+  /** Return the {@link DnsWriter} for the given tld. */
+  public DnsWriter getForTld(String tld) {
+    String clazz = Registry.get(tld).getDnsWriter();
+    DnsWriter dnsWriter = dnsWriters.get(clazz);
+    checkState(dnsWriter != null, "Could not load DnsWriter %s for TLD %s", clazz, tld);
+    return dnsWriter;
+  }
+}