Initial implementation of a com.sun.source.util.Plugin

This allows Error Prone to be used as a javac plugin (instead of a
wrapper) via the plugin API that was added in 8:
https://docs.oracle.com/javase/8/docs/jdk/api/javac/tree/com/sun/source/util/Plugin.html

Demo:

```
javac -J-Xbootclasspath/p:javac-9-dev-r3297-3.jar \
  -processorpath error_prone_ant-2.0.18-SNAPSHOT.jar \
  -Xplugin:ErrorProne Test.java
...
ShortSet.java:8: error: [CollectionIncompatibleType] Argument 'i - 1' should not be passed to this method; its type int is not compatible with its collection's type argument Short
      s.remove(i - 1);
              ^
    (see http://errorprone.info/bugpattern/CollectionIncompatibleType)
```

See #535

MOE_MIGRATED_REVID=148835033

check_api/src/main/java/com/google/errorprone/BaseErrorProneCompiler.java

@@ -37,6 +37,7 @@
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
+import java.util.ResourceBundle;
 import java.util.Set;
 import javax.annotation.Nullable;
 import javax.annotation.processing.Processor;
@@ -293,10 +294,9 @@ private Result wrapPotentialRefactoringCall(
     }
   }
 
-  /**
-   * Registers our message bundle.
-   */
+  /** Registers our message bundle. */
   public static void setupMessageBundle(Context context) {
-    JavacMessages.instance(context).add("com.google.errorprone.errors");
+    ResourceBundle bundle = ResourceBundle.getBundle("com.google.errorprone.errors");
+    JavacMessages.instance(context).add(l -> bundle);
   }
 }

core/pom.xml

@@ -267,6 +267,13 @@
       <version>r6</version>
       <scope>test</scope>
     </dependency>
+    <dependency>
+      <!-- Apache 2.0 -->
+      <groupId>com.google.auto.service</groupId>
+      <artifactId>auto-service</artifactId>
+      <version>1.0-rc2</version>
+      <scope>provided</scope>
+    </dependency>
   </dependencies>
 
   <build>

core/src/main/java/com/google/errorprone/ErrorProneJavacPlugin.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright 2017 Google Inc. 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 com.google.errorprone;
+
+import com.google.auto.service.AutoService;
+import com.google.errorprone.scanner.BuiltInCheckerSuppliers;
+import com.sun.source.util.JavacTask;
+import com.sun.source.util.Plugin;
+import com.sun.tools.javac.api.BasicJavacTask;
+import com.sun.tools.javac.util.Context;
+
+/** A javac {@link Plugin} that runs Error Prone. */
+@AutoService(Plugin.class)
+public class ErrorProneJavacPlugin implements Plugin {
+  @Override
+  public String getName() {
+    return "ErrorProne";
+  }
+
+  @Override
+  public void init(JavacTask javacTask, String... args) {
+    Context context = ((BasicJavacTask) javacTask).getContext();
+    BaseErrorProneCompiler.setupMessageBundle(context);
+    javacTask.addTaskListener(
+        ErrorProneAnalyzer.createByScanningForPlugins(
+            BuiltInCheckerSuppliers.defaultChecks(), ErrorProneOptions.processArgs(args), context));
+  }
+}

core/src/test/java/com/google/errorprone/ErrorProneJavacPluginTest.java

@@ -0,0 +1,85 @@
+/*
+ * Copyright 2017 Google Inc. 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 com.google.errorprone;
+
+import static com.google.common.collect.MoreCollectors.onlyElement;
+import static com.google.common.truth.Truth.assertThat;
+import static java.nio.charset.StandardCharsets.UTF_8;
+import static java.util.Locale.ENGLISH;
+
+import com.google.common.collect.ImmutableList;
+import com.google.common.jimfs.Configuration;
+import com.google.common.jimfs.Jimfs;
+import com.sun.source.util.JavacTask;
+import com.sun.tools.javac.api.JavacTool;
+import com.sun.tools.javac.file.JavacFileManager;
+import com.sun.tools.javac.util.Context;
+import java.io.IOException;
+import java.nio.file.FileSystem;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import javax.tools.Diagnostic;
+import javax.tools.DiagnosticCollector;
+import javax.tools.JavaFileObject;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** {@link ErrorProneJavacPlugin}Test */
+@RunWith(JUnit4.class)
+public class ErrorProneJavacPluginTest {
+  @Test
+  public void hello() throws IOException {
+    FileSystem fileSystem = Jimfs.newFileSystem(Configuration.unix());
+    Path source = fileSystem.getPath("Test.java");
+    Files.write(
+        source,
+        ImmutableList.of(
+            "import java.util.HashSet;",
+            "import java.util.Set;",
+            "class Test {",
+            "  public static void main(String[] args) {",
+            "    Set<Short> s = new HashSet<>();",
+            "    for (short i = 0; i < 100; i++) {",
+            "      s.add(i);",
+            "      s.remove(i - 1);",
+            "    }",
+            "    System.out.println(s.size());",
+            "  }",
+            "}"),
+        UTF_8);
+    JavacFileManager fileManager = new JavacFileManager(new Context(), false, UTF_8);
+    DiagnosticCollector<JavaFileObject> diagnosticCollector = new DiagnosticCollector<>();
+    JavacTask task =
+        JavacTool.create()
+            .getTask(
+                null,
+                fileManager,
+                diagnosticCollector,
+                ImmutableList.of("-Xplugin:ErrorProne"),
+                ImmutableList.of(),
+                fileManager.getJavaFileObjects(source));
+    assertThat(task.call()).isFalse();
+    Diagnostic<? extends JavaFileObject> diagnostic =
+        diagnosticCollector
+            .getDiagnostics()
+            .stream()
+            .filter(d -> d.getKind() == Diagnostic.Kind.ERROR)
+            .collect(onlyElement());
+    assertThat(diagnostic.getMessage(ENGLISH)).contains("[CollectionIncompatibleType]");
+  }
+}