Add unitAboutToCompile() into DartCompilerListener

compiler/javatests/com/google/dart/compiler/parser/SyntaxTest.java

Index: compiler/javatests/com/google/dart/compiler/parser/SyntaxTest.java
diff --git a/compiler/javatests/com/google/dart/compiler/parser/SyntaxTest.java b/compiler/javatests/com/google/dart/compiler/parser/SyntaxTest.java
index 8f35627a7ac4b1194966f8e91e43559a98662d86..72918e305721d4f499003a219f9530e3a2595a9c 100644
--- a/compiler/javatests/com/google/dart/compiler/parser/SyntaxTest.java
+++ b/compiler/javatests/com/google/dart/compiler/parser/SyntaxTest.java
@@ -5,7 +5,6 @@
 package com.google.dart.compiler.parser;
 
 import com.google.common.base.Joiner;
-import com.google.dart.compiler.DartCompilationError;
 import com.google.dart.compiler.DartCompilerListener;
 import com.google.dart.compiler.DartSourceTest;
 import com.google.dart.compiler.ast.DartArrayLiteral;
@@ -108,14 +107,7 @@ public class SyntaxTest extends AbstractParserTest {
     try {
       DartSourceTest dartSrc = new DartSourceTest(getName(), sourceCode, null);
       DartScannerParserContext context =
-        new DartScannerParserContext(dartSrc, sourceCode, new DartCompilerListener() {
-        @Override
-        public void onError(DartCompilationError event) {
-        }
-        @Override
-        public void unitCompiled(DartUnit unit) {
-        }
-      });
+        new DartScannerParserContext(dartSrc, sourceCode, new DartCompilerListener.Empty());
       DartParser parser = new DartParser(context);
       parser.parseExpression();
     }