Fix for parsing empty compilation units with Fasta.

R=paulberry@google.com
BUG=

Review-Url: https://codereview.chromium.org/2738693003 .

pkg/analyzer/test/generated/parser_fasta_test.dart

@@ -1240,13 +1240,6 @@ class TopLevelParserTest_Fasta extends FastaParserTestCase
     super.test_parseCompilationUnit_builtIn_asFunctionName();
   }
 
-  @override
-  @failingTest
-  void test_parseCompilationUnit_empty() {
-    // TODO(paulberry): No objects placed on stack
-    super.test_parseCompilationUnit_empty();
-  }
-
   @override
   @failingTest
   void test_parseCompilationUnit_exportAsPrefix() {
@@ -1297,13 +1290,6 @@ class TopLevelParserTest_Fasta extends FastaParserTestCase
     super.test_parseDirectives_complete();
   }
 
-  @override
-  @failingTest
-  void test_parseDirectives_empty() {
-    // TODO(paulberry): No objects placed on stack
-    super.test_parseDirectives_empty();
-  }
-
   @override
   @failingTest
   void test_parseDirectives_mixed() {

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

@@ -852,14 +852,17 @@ class AstBuilder extends ScopeListener {
     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(