Add unitAboutToCompile() into DartCompilerListener

compiler/java/com/google/dart/compiler/DeltaAnalyzer.java

Index: compiler/java/com/google/dart/compiler/DeltaAnalyzer.java
diff --git a/compiler/java/com/google/dart/compiler/DeltaAnalyzer.java b/compiler/java/com/google/dart/compiler/DeltaAnalyzer.java
index 8625698bb3a6cb7a692e6635b070afbfa5313a2a..7e827c5cf92cd5afa97300bfeff3569897bc91a9 100644
--- a/compiler/java/com/google/dart/compiler/DeltaAnalyzer.java
+++ b/compiler/java/com/google/dart/compiler/DeltaAnalyzer.java
@@ -141,7 +141,7 @@ class DeltaAnalyzer {
     return new DartParser(new DartScannerParserContext(source, sourceString, listener), false);
   }
 
-  private class Context extends DartCompilerListener implements DartCompilerContext {
+  private class Context implements DartCompilerListener, DartCompilerContext {
     @Override
     public LibraryUnit getApplicationUnit() {
       throw new AssertionError();
@@ -203,6 +203,10 @@ class DeltaAnalyzer {
     }
 
     @Override
+    public void unitAboutToCompile(DartSource source, boolean diet) {
+    }
+
+    @Override
     public void unitCompiled(DartUnit unit) {
     }
   }