Fix for parsing empty compilation units with Fasta.

pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.analyzer.ast_builder;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/dart/ast/ast_factory.dart' show AstFactory;
 import 'package:analyzer/dart/ast/standard_ast_factory.dart' as standard;
 import 'package:analyzer/dart/ast/token.dart' as analyzer show Token;
@@ skipping 834 matching lines @@
   }
 
   @override
   void endCompilationUnit(int count, Token token) {
     debugEvent("CompilationUnit");
     analyzer.Token beginToken = null; // TODO(paulberry)
     ScriptTag scriptTag = null; // TODO(paulberry)
     var directives = <Directive>[];
     var declarations = <CompilationUnitMember>[];
     analyzer.Token endToken = null; // TODO(paulberry)
-    for (AstNode node in popList(count)) {
-      if (node is Directive) {
-        directives.add(node);
-      } else if (node is CompilationUnitMember) {
-        declarations.add(node);
-      } else {
-        internalError(
-            'Unrecognized compilation unit member: ${node.runtimeType}');
+    List<Object> elements = popList(count);
+    if (elements != null) {
+      for (AstNode node in elements) {
+        if (node is Directive) {
+          directives.add(node);
+        } else if (node is CompilationUnitMember) {
+          declarations.add(node);
+        } else {
+          internalError(
+              'Unrecognized compilation unit member: ${node.runtimeType}');
+        }
       }
     }
     push(ast.compilationUnit(
         beginToken, scriptTag, directives, declarations, endToken));
   }
 
   void endImport(Token importKeyword, Token deferredKeyword, Token asKeyword,
       Token semicolon) {
     debugEvent("Import");
     List<Combinator> combinators = pop();
@@ skipping 607 matching lines @@
       } else if (identical('static', s)) {
         staticKeyword = token;
       } else if (identical('var', s)) {
         finalConstOrVarKeyword = token;
       } else {
         internalError('Unhandled modifier: $s');
       }
     }
   }
 }