Use NodeList where possible.

compiler/java/com/google/dart/compiler/ast/DartUnit.java

Index: compiler/java/com/google/dart/compiler/ast/DartUnit.java
diff --git a/compiler/java/com/google/dart/compiler/ast/DartUnit.java b/compiler/java/com/google/dart/compiler/ast/DartUnit.java
index 01c6b3df71f6463fe561166c472cf5f8b138d72f..fabd1aca4629bf334c789b6add249d1c81b4d4ac 100644
--- a/compiler/java/com/google/dart/compiler/ast/DartUnit.java
+++ b/compiler/java/com/google/dart/compiler/ast/DartUnit.java
@@ -4,12 +4,9 @@
 
 package com.google.dart.compiler.ast;
 
-import com.google.common.collect.Lists;
 import com.google.common.collect.Sets;
 import com.google.dart.compiler.DartSource;
 
-import java.util.ArrayList;
-import java.util.Collections;
 import java.util.List;
 import java.util.Set;
 
@@ -21,22 +18,17 @@ public class DartUnit extends DartNode {
   private static final long serialVersionUID = -3407637869012712127L;
 
   private LibraryUnit library;
-  private List<DartDirective> directives;
-  private final List<DartNode> topLevelNodes = Lists.newArrayList();
+  private final NodeList<DartDirective> directives = NodeList.create(this);
+  private final NodeList<DartNode> topLevelNodes = NodeList.create(this);
+  private final NodeList<DartComment> comments = NodeList.create(this);
   private final DartSource source;
   private final boolean isDiet;
-  /** A list of comments. May be null. */
-  private List<DartComment> comments;
 
   public DartUnit(DartSource source, boolean isDiet) {
     this.source = source;
     this.isDiet = isDiet;
   }
 
-  public void addTopLevelNode(DartNode node) {
-    topLevelNodes.add(becomeParentOf(node));
-  }
-
   public String getSourceName() {
     return source.getName();
   }
@@ -46,19 +38,8 @@ public class DartUnit extends DartNode {
     return source;
   }
 
-  public void addComment(DartComment comment) {
-    if (comments == null) {
-      comments = new ArrayList<DartComment>();
-    }
-    comments.add(becomeParentOf(comment));
-  }
-
   public List<DartComment> getComments() {
-    return comments == null ? null : Collections.unmodifiableList(comments);
-  }
-
-  public boolean removeComment(DartComment comment) {
-    return comments == null ? false : comments.remove(comment);
+    return comments;
   }
 
   public void setLibrary(LibraryUnit library) {
@@ -75,13 +56,9 @@ public class DartUnit extends DartNode {
 
   @Override
   public void visitChildren(ASTVisitor<?> visitor) {
-    if (directives != null) {
-      visitor.visit(directives);
-    }
-    visitor.visit(topLevelNodes);
-    if (comments != null) {
-      visitor.visit(comments);
-    }
+    directives.accept(visitor);
+    topLevelNodes.accept(visitor);
+    comments.accept(visitor);
   }
 
   @Override
@@ -97,22 +74,9 @@ public class DartUnit extends DartNode {
   }
 
   /**
-   * Add the specified directive to the receiver's list of directives
-   */
-  public void addDirective(DartDirective directive) {
-    if (directives == null) {
-      directives = new ArrayList<DartDirective>();
-    }
-    directives.add(becomeParentOf(directive));
-  }
-
-  /**
-   * Answer the receiver's directives or <code>null</code> if none
+   * Answer the receiver's directives, not <code>null</code>.
    */
   public List<DartDirective> getDirectives() {
-    if (directives == null) {
-      return Collections.<DartDirective> emptyList();
-    }
     return directives;
   }