Fix some crashes.

pkg/front_end/lib/src/fasta/source/source_loader.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.source_loader;
 
 import 'dart:async' show Future;
 
-import 'dart:io' show FileSystemException;
-
 import 'dart:typed_data' show Uint8List;
 
 import 'package:kernel/ast.dart' show Program;
 
 import 'package:kernel/class_hierarchy.dart' show ClassHierarchy;
 
 import 'package:kernel/core_types.dart' show CoreTypes;
 
 import '../builder/builder.dart' show Builder, ClassBuilder, LibraryBuilder;
 
 import '../compiler_context.dart' show CompilerContext;
 
 import '../errors.dart' show inputError;
 
 import '../export.dart' show Export;
 
 import '../loader.dart' show Loader;
 
 import '../parser/class_member_parser.dart' show ClassMemberParser;
 
 import '../scanner.dart' show ErrorToken, ScannerResult, Token, scan;
 
-import '../scanner/io.dart' show readBytesFromFile;
+import '../io.dart' show readBytesFromFile;
 
 import '../target_implementation.dart' show TargetImplementation;
 
 import 'diet_listener.dart' show DietListener;
 
 import 'diet_parser.dart' show DietParser;
 
 import 'outline_builder.dart' show OutlineBuilder;
 
 import 'source_class_builder.dart' show SourceClassBuilder;
 
 import 'source_library_builder.dart' show SourceLibraryBuilder;
 
 class SourceLoader<L> extends Loader<L> {
   final Map<Uri, List<int>> sourceBytes = <Uri, List<int>>{};
   final bool excludeSource = CompilerContext.current.options.excludeSource;
 
   // Used when building directly to kernel.
   ClassHierarchy hierarchy;
   CoreTypes coreTypes;
 
   SourceLoader(TargetImplementation target) : super(target);
 
   Future<Token> tokenize(SourceLibraryBuilder library,
       {bool suppressLexicalErrors: false}) async {
     Uri uri = library.fileUri;
     if (uri == null || uri.scheme != "file") {
       return inputError(library.uri, -1, "Not found: ${library.uri}.");
     }
-    try {
-      List<int> bytes = sourceBytes[uri];
-      if (bytes == null) {
-        bytes = sourceBytes[uri] = await readBytesFromFile(uri);
+    List<int> bytes = sourceBytes[uri];
+    if (bytes == null) {
+      bytes = sourceBytes[uri] = await readBytesFromFile(uri);
+    }
+    byteCount += bytes.length - 1;
+    ScannerResult result = scan(bytes);
+    Token token = result.tokens;
+    if (!suppressLexicalErrors) {
+      List<int> source = getSource(bytes);
+      target.addSourceInformation(library.fileUri, result.lineStarts, source);
+    }
+    while (token is ErrorToken) {
+      if (!suppressLexicalErrors) {
+        ErrorToken error = token;
+        library.addCompileTimeError(token.charOffset, error.assertionMessage,
+            fileUri: uri);
       }
-      byteCount += bytes.length - 1;
-      ScannerResult result = scan(bytes);
-      Token token = result.tokens;
-      if (!suppressLexicalErrors) {
-        List<int> source = getSource(bytes);
-        target.addSourceInformation(library.fileUri, result.lineStarts, source);
-      }
-      while (token is ErrorToken) {
-        if (!suppressLexicalErrors) {
-          ErrorToken error = token;
-          library.addCompileTimeError(token.charOffset, error.assertionMessage,
-              fileUri: uri);
-        }
-        token = token.next;
-      }
-      return token;
-    } on FileSystemException catch (e) {
-      String message = e.message;
-      String osMessage = e.osError?.message;
-      if (osMessage != null && osMessage.isNotEmpty) {
-        message = osMessage;
-      }
-      return inputError(uri, -1, message);
+      token = token.next;
     }
+    return token;
   }
 
   List<int> getSource(List<int> bytes) {
     if (excludeSource) return const <int>[];
 
     // bytes is 0-terminated. We don't want that included.
     if (bytes is Uint8List) {
       return new Uint8List.view(
           bytes.buffer, bytes.offsetInBytes, bytes.length - 1);
     }
@@ skipping 260 matching lines @@
   void checkOverrides(List<SourceClassBuilder> sourceClasses) {
     assert(hierarchy != null);
     for (SourceClassBuilder builder in sourceClasses) {
       builder.checkOverrides(hierarchy);
     }
     ticker.logMs("Checked overrides");
   }
 
   List<Uri> getDependencies() => sourceBytes.keys.toList();
 }