Add unitAboutToCompile() into DartCompilerListener

compiler/javatests/com/google/dart/compiler/CompilerTestCase.java

Index: compiler/javatests/com/google/dart/compiler/CompilerTestCase.java
diff --git a/compiler/javatests/com/google/dart/compiler/CompilerTestCase.java b/compiler/javatests/com/google/dart/compiler/CompilerTestCase.java
index ca89628071d860791c0c1acab323b799acefdf7f..02f7c585394122a9230030383fd1318efd4f880a 100644
--- a/compiler/javatests/com/google/dart/compiler/CompilerTestCase.java
+++ b/compiler/javatests/com/google/dart/compiler/CompilerTestCase.java
@@ -83,7 +83,7 @@ public abstract class CompilerTestCase extends TestCase {
   /**
    * Collects the results of running analyzeLibrary.
    */
-  protected static class AnalyzeLibraryResult extends DartCompilerListener {
+  protected static class AnalyzeLibraryResult extends DartCompilerListener.Empty {
     private final List<DartCompilationError> compilationErrors;
     private final List<DartCompilationError> compilationWarnings;
     private final List<DartCompilationError> typeErrors;
@@ -131,10 +131,6 @@ public abstract class CompilerTestCase extends TestCase {
     public LibraryUnit getLibraryUnitResult() {
       return result;
     }
-
-    @Override
-    public void unitCompiled(DartUnit unit) {
-    }
   }
 
   /**