Revert "Remove check for scheme in source-loader."

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:typed_data' show Uint8List;
 
@@ skipping 16 matching lines @@
         ClassBuilder,
         EnumBuilder,
         LibraryBuilder,
         NamedTypeBuilder,
         TypeBuilder;
 
 import '../compiler_context.dart' show CompilerContext;
 
 import '../deprecated_problems.dart' show deprecated_inputError;
 
-import '../problems.dart' show internalProblem;
-
 import '../export.dart' show Export;
 
 import '../fasta_codes.dart'
     show
         Message,
         templateCyclicClassHierarchy,
         templateExtendingEnum,
         templateExtendingRestricted,
         templateIllegalMixin,
         templateIllegalMixinDueToConstructors,
         templateIllegalMixinDueToConstructorsCause,
-        templateInternalProblemUriMissingScheme,
         templateUnspecified;
 
 import '../kernel/kernel_shadow_ast.dart' show KernelTypeInferenceEngine;
 
 import '../kernel/kernel_target.dart' show KernelTarget;
 
 import '../loader.dart' show Loader;
 
 import '../parser/class_member_parser.dart' show ClassMemberParser;
 
@@ skipping 31 matching lines @@
   TypeInferenceEngine typeInferenceEngine;
 
   Instrumentation instrumentation;
 
   SourceLoader(this.fileSystem, this.includeComments, KernelTarget target)
       : super(target);
 
   Future<Token> tokenize(SourceLibraryBuilder library,
       {bool suppressLexicalErrors: false}) async {
     Uri uri = library.fileUri;
-    if (uri == null) {
+    // TODO(sigmund): source-loader shouldn't check schemes, but defer to the
+    // underlying file system to decide whether it is supported.
+    if (uri == null || uri.scheme != "file" && uri.scheme != "multi-root") {
       return deprecated_inputError(
           library.uri, -1, "Not found: ${library.uri}.");
-    } else if (!uri.hasScheme) {
-      return internalProblem(
-          templateInternalProblemUriMissingScheme.withArguments(uri),
-          -1,
-          library.uri);
     }
 
     // Get the library text from the cache, or read from the file system.
     List<int> bytes = sourceBytes[uri];
     if (bytes == null) {
       try {
         List<int> rawBytes = await fileSystem.entityForUri(uri).readAsBytes();
         Uint8List zeroTerminatedBytes = new Uint8List(rawBytes.length + 1);
         zeroTerminatedBytes.setRange(0, rawBytes.length, rawBytes);
         bytes = zeroTerminatedBytes;
@@ skipping 432 matching lines @@
   Expression throwCompileConstantError(Expression error) {
     return target.backendTarget.throwCompileConstantError(coreTypes, error);
   }
 
   Expression buildCompileTimeError(Message message, int offset, Uri uri) {
     String text = target.context
         .format(message.withLocation(uri, offset), Severity.error);
     return target.backendTarget.buildCompileTimeError(coreTypes, text, offset);
   }
 }