Correctly recover from incorrect imports, etc.

pkg/front_end/lib/src/fasta/source/outline_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.outline_builder;
 
 import 'package:kernel/ast.dart' show AsyncMarker, ProcedureKind;
 
 import '../parser/parser.dart' show FormalParameterType, optional;
 
@@ skipping 103 matching lines @@
     push(popList(count) ?? NullValue.Combinators);
   }
 
   @override
   void endExport(Token exportKeyword, Token semicolon) {
     debugEvent("Export");
     List<Combinator> combinators = pop();
     Unhandled conditionalUris = pop();
     String uri = pop();
     List<MetadataBuilder> metadata = pop();
-    library.addExport(
-        metadata, uri, conditionalUris, combinators, exportKeyword.charOffset);
+    if (uri != null) {
+      library.addExport(metadata, uri, conditionalUris, combinators,
+          exportKeyword.charOffset);
+    }
     checkEmpty(exportKeyword.charOffset);
   }
 
   @override
   void endImport(Token importKeyword, Token deferredKeyword, Token asKeyword,
       Token semicolon) {
     debugEvent("endImport");
     List<Combinator> combinators = pop();
     String prefix = popIfNotNull(asKeyword);
     Unhandled conditionalUris = pop();
     String uri = pop();
     List<MetadataBuilder> metadata = pop();
-    library.addImport(
-        metadata,
-        uri,
-        conditionalUris,
-        prefix,
-        combinators,
-        deferredKeyword != null,
-        importKeyword.charOffset,
-        asKeyword?.next?.charOffset ?? -1);
+    if (uri != null) {
+      library.addImport(
+          metadata,
+          uri,
+          conditionalUris,
+          prefix,
+          combinators,
+          deferredKeyword != null,
+          importKeyword.charOffset,
+          asKeyword?.next?.charOffset ?? -1);
+    }
     checkEmpty(importKeyword.charOffset);
   }
 
   @override
+  void handleRecoverExpression(Token token) {
+    debugEvent("RecoverExpression");
+    push(NullValue.Expression);
+  }
+
+  @override
   void endPart(Token partKeyword, Token semicolon) {
     debugEvent("Part");
     String uri = pop();
     List<MetadataBuilder> metadata = pop();
-    library.addPart(metadata, uri);
+    if (uri != null) {
+      library.addPart(metadata, uri);
+    }
     checkEmpty(partKeyword.charOffset);
   }
 
   @override
   void handleOperatorName(Token operatorKeyword, Token token) {
     debugEvent("OperatorName");
     push(operatorFromString(token.stringValue));
   }
 
   @override
@@ skipping 459 matching lines @@
   Link<Token> handleMemberName(Link<Token> identifiers) {
     if (!isDartLibrary || identifiers.isEmpty) return identifiers;
     return removeNativeClause(identifiers);
   }
 
   @override
   void debugEvent(String name) {
     // printEvent(name);
   }
 }