Add unitAboutToCompile() into DartCompilerListener

compiler/java/com/google/dart/compiler/parser/ParserContext.java

Index: compiler/java/com/google/dart/compiler/parser/ParserContext.java
diff --git a/compiler/java/com/google/dart/compiler/parser/ParserContext.java b/compiler/java/com/google/dart/compiler/parser/ParserContext.java
index 94b15bfff02ebe0931ec3bafa3e0913387dd55d1..535e999f09447432e1a8841861599cd928ed147b 100644
--- a/compiler/java/com/google/dart/compiler/parser/ParserContext.java
+++ b/compiler/java/com/google/dart/compiler/parser/ParserContext.java
@@ -5,6 +5,7 @@
 package com.google.dart.compiler.parser;
 
 import com.google.dart.compiler.DartCompilationError;
+import com.google.dart.compiler.DartSource;
 import com.google.dart.compiler.Source;
 
 /**
@@ -75,6 +76,11 @@ public interface ParserContext {
   void error(DartCompilationError dartError);
 
   /**
+   * Called by the {@link DartParser} before parsing given {@link DartSource}.
+   */
+  void unitAboutToCompile(DartSource source, boolean diet);
+
+  /**
    * Return the current token.
    */
   Token getCurrentToken();