Add unitAboutToCompile() into DartCompilerListener

compiler/java/com/google/dart/compiler/DartCompilerListener.java

Index: compiler/java/com/google/dart/compiler/DartCompilerListener.java
diff --git a/compiler/java/com/google/dart/compiler/DartCompilerListener.java b/compiler/java/com/google/dart/compiler/DartCompilerListener.java
index a182a35112998385124ec1a423571c7c2f66e19f..9216c5bced2b2d0120eef610c748b2db5080516c 100644
--- a/compiler/java/com/google/dart/compiler/DartCompilerListener.java
+++ b/compiler/java/com/google/dart/compiler/DartCompilerListener.java
@@ -1,29 +1,53 @@
 // Copyright (c) 2011, 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.
-
 package com.google.dart.compiler;
 
 import com.google.dart.compiler.ast.DartUnit;
 
 /**
- * Abstract class that {@link DartCompiler} consumers can use to monitor
- * compilation progress and report various problems that occur during
- * compilation.
+ * Interface that {@link DartCompiler} consumers can use to monitor compilation progress and report
+ * various problems that occur during compilation.
  */
-public abstract class DartCompilerListener {
-
+public interface DartCompilerListener {
   /**
    * Called by the compiler when a compilation error has occurred in a Dart file.
-   *
+   * 
    * @param event the event information (not <code>null</code>)
    */
-  public abstract void onError(DartCompilationError event);
+  void onError(DartCompilationError event);
+
+  /**
+   * Called by the compiler before parsing given {@link DartSource}.
+   */
+  void unitAboutToCompile(DartSource source, boolean diet);
 
   /**
    * Called by the compiler after the resolution and type analyzer phase for each unit.
-   *
-   * @param unit the DartUnit having just been compiled. (not <code>null</code>)
+   * 
+   * @param unit the {@link DartUnit} having just been compiled (not <code>null</code>)
+   */
+  void unitCompiled(DartUnit unit);
+
+  /**
+   * Implementation of {@link DartCompilerListener} which does nothing.
+   */
+  public static class Empty implements DartCompilerListener {
+    @Override
+    public void onError(DartCompilationError event) {
+    }
+
+    @Override
+    public void unitAboutToCompile(DartSource source, boolean diet) {
+    }
+
+    @Override
+    public void unitCompiled(DartUnit unit) {
+    }
+  }
+  
+  /**
+   * Instance of {@link DartCompilerListener} which does nothing.
    */
-  public abstract void unitCompiled(DartUnit unit);
+  public static final DartCompilerListener EMPTY = new Empty();
 }