Add unitAboutToCompile() into DartCompilerListener

editor/tools/plugins/com.google.dart.tools.core/src/com/google/dart/tools/core/internal/builder/CompilerListener.java

Index: editor/tools/plugins/com.google.dart.tools.core/src/com/google/dart/tools/core/internal/builder/CompilerListener.java
diff --git a/editor/tools/plugins/com.google.dart.tools.core/src/com/google/dart/tools/core/internal/builder/CompilerListener.java b/editor/tools/plugins/com.google.dart.tools.core/src/com/google/dart/tools/core/internal/builder/CompilerListener.java
index 17b82a1121027e6649f188f24ae069dd80e6acc0..2564848499aec57b18943f17beb5fdf0d074d316 100644
--- a/editor/tools/plugins/com.google.dart.tools.core/src/com/google/dart/tools/core/internal/builder/CompilerListener.java
+++ b/editor/tools/plugins/com.google.dart.tools.core/src/com/google/dart/tools/core/internal/builder/CompilerListener.java
@@ -45,7 +45,7 @@ import java.net.URI;
  * An Eclipse specific implementation of {@link DartCompilerContext} for intercepting compilation
  * errors and translating them into {@link IResource} markers.
  */
-class CompilerListener extends DartCompilerListener {
+class CompilerListener implements DartCompilerListener {
   /**
    * The number of times that we have logged a message about compilation errors that were reported
    * for which we could not associate a source file and were forced to associate the marker with the
@@ -87,6 +87,10 @@ class CompilerListener extends DartCompilerListener {
   }
 
   @Override
+  public void unitAboutToCompile(DartSource source, boolean diet) {
+  }
+
+  @Override
   public void unitCompiled(DartUnit unit) {
     if (unit.isDiet()) {
       return;