Add unitAboutToCompile() into DartCompilerListener

editor/tools/plugins/com.google.dart.tools.core/src/com/google/dart/tools/core/internal/formatter/CodeSnippetParsingUtil.java

Index: editor/tools/plugins/com.google.dart.tools.core/src/com/google/dart/tools/core/internal/formatter/CodeSnippetParsingUtil.java
diff --git a/editor/tools/plugins/com.google.dart.tools.core/src/com/google/dart/tools/core/internal/formatter/CodeSnippetParsingUtil.java b/editor/tools/plugins/com.google.dart.tools.core/src/com/google/dart/tools/core/internal/formatter/CodeSnippetParsingUtil.java
index dc243359b5a5be69d7a136c4d1d6f3561c60bf01..66aa0489b9761e0daea3a5353e7a647c81c2e014 100644
--- a/editor/tools/plugins/com.google.dart.tools.core/src/com/google/dart/tools/core/internal/formatter/CodeSnippetParsingUtil.java
+++ b/editor/tools/plugins/com.google.dart.tools.core/src/com/google/dart/tools/core/internal/formatter/CodeSnippetParsingUtil.java
@@ -132,15 +132,11 @@ public class CodeSnippetParsingUtil {
     String sourceCode = new String(source);
     final CompilationResult compilationResult = new CompilationResult();
     compilationResult.scanLines(sourceCode);
-    DartCompilerListener listener = new DartCompilerListener() {
+    DartCompilerListener listener = new DartCompilerListener.Empty() {
       @Override
       public void onError(DartCompilationError event) {
         compilationResult.problemCount += 1;
       }
-
-      @Override
-      public void unitCompiled(DartUnit unit) {
-      }
     };
     DartScannerParserContext ctx = new DartScannerParserContext(null, sourceCode, listener);
     CommentPreservingParser parser = new CommentPreservingParser(sourceCode, listener, false);