diff --git a/pkg/front_end/lib/compiler_options.dart b/pkg/front_end/lib/compiler_options.dart
index 5230f4e47fb..731344c03dc 100644
--- a/pkg/front_end/lib/compiler_options.dart
+++ b/pkg/front_end/lib/compiler_options.dart
@@ -18,13 +18,13 @@ typedef void ErrorHandler(CompilationError error);
 ///
 /// Not intended to be implemented or extended by clients.
 class CompilerOptions {
-  /// The path to the Dart SDK.
+  /// The URI of the root of the Dart SDK (typically a "file:" URI).
   ///
   /// If `null`, the SDK will be searched for using
   /// [Platform.resolvedExecutable] as a starting point.
   ///
   /// This option is mutually exclusive with [sdkSummary].
-  String sdkPath;
+  Uri sdkRoot;
 
   /// Callback to which compilation errors should be delivered.
   ///
@@ -32,56 +32,58 @@ class CompilerOptions {
   /// type [CompilationError].
   ErrorHandler onError = defaultErrorHandler;
 
-  /// Path to the ".packages" file.
+  /// URI of the ".packages" file (typically a "file:" URI).
   ///
   /// If `null`, the ".packages" file will be found via the standard
   /// package_config search algorithm.
   ///
-  /// If the empty string, no packages file will be used.
-  String packagesFilePath;
+  /// If the URI's path component is empty (e.g. `new Uri()`), no packages file
+  /// will be used.
+  Uri packagesFileUri;
 
-  /// Paths to the input summary files (excluding the SDK summary).  These files
-  /// should all be linked summaries.  They should also be closed, in the sense
-  /// that any libraries they reference should also appear in [inputSummaries]
-  /// or [sdkSummary].
-  List<String> inputSummaries = [];
+  /// URIs of input summary files (excluding the SDK summary; typically these
+  /// will be "file:" URIs).  These files should all be linked summaries.  They
+  /// should also be closed, in the sense that any libraries they reference
+  /// should also appear in [inputSummaries] or [sdkSummary].
+  List<Uri> inputSummaries = [];
 
-  /// Path to the SDK summary file.
+  /// URI of the SDK summary file (typically a "file:" URI).
   ///
   /// This should be a linked summary.  If `null`, the SDK summary will be
-  /// searched for at a default location within [sdkPath].
+  /// searched for at a default location within [sdkRoot].
   ///
-  /// This option is mutually exclusive with [sdkPath].  TODO(paulberry): if the
+  /// This option is mutually exclusive with [sdkRoot].  TODO(paulberry): if the
   /// VM does not contain a pickled copy of the SDK, we might need to change
   /// this.
-  String sdkSummary;
+  Uri sdkSummary;
 
   /// URI override map.
   ///
-  /// This is a map from Uri to file path.  Any URI override listed in this map
-  /// takes precedence over the URI resolution that would be implied by the
-  /// packages file (see [packagesFilePath]) and/or [bazelRoots].
+  /// This is a map from URIs that might appear in import/export/part statements
+  /// to URIs that should be used to locate the corresponding files in the
+  /// [fileSystem].  Any URI override listed in this map takes precedence over
+  /// the URI resolution that would be implied by the packages file (see
+  /// [packagesFileUri]) and/or [multiRoots].
   ///
   /// If a URI is not listed in this map, then the normal URI resolution
   /// algorithm will be used.
   ///
   /// TODO(paulberry): transition analyzer and dev_compiler to use the
-  /// "file:///bazel-root" mechanism, and then remove this.
+  /// "multi-root:" mechanism, and then remove this.
   @deprecated
-  Map<Uri, String> uriOverride = {};
+  Map<Uri, Uri> uriOverride = {};
 
-  /// Bazel roots.
+  /// Multi-roots.
   ///
-  /// Any Uri that resolves to "file:///bazel-root/$rest" will be searched for
-  /// at "$root/$rest" ("$root\\$rest" in Windows), where "$root" is drawn from
-  /// this list.  If the file is not found at any of those locations, the URI
-  /// "file:///bazel-root/$rest" will be used directly.
+  /// Any Uri that resolves to "multi-root:///$rest" will be searched for
+  /// at "$root/$rest", where "$root" is drawn from this list.
   ///
-  /// Intended use: if the Bazel workspace is located at path "$workspace", this
-  /// could be set to `['$workspace', '$workspace/bazel-bin',
-  /// '$workspace/bazel-genfiles']`, effectively overlaying source and generated
-  /// files.
-  List<String> bazelRoots = [];
+  /// Intended use: if the user has a Bazel workspace located at path
+  /// "$workspace", this could be set to the file URIs corresponding to the
+  /// paths for "$workspace", "$workspace/bazel-bin",
+  /// and "$workspace/bazel-genfiles", effectively overlaying source and
+  /// generated files.
+  List<Uri> multiRoots = [];
 
   /// Sets the platform bit, which determines which patch files should be
   /// applied to the SDK.
@@ -99,7 +101,7 @@ class CompilerOptions {
   ///
   /// All file system access performed by the front end goes through this
   /// mechanism, with one exception: if no value is specified for
-  /// [packagesFilePath], the packages file is located using the actual physical
+  /// [packagesFileUri], the packages file is located using the actual physical
   /// file system.  TODO(paulberry): fix this.
   FileSystem fileSystem = PhysicalFileSystem.instance;
 
diff --git a/pkg/front_end/lib/file_system.dart b/pkg/front_end/lib/file_system.dart
index cadf82fddf4..6e57c266c06 100644
--- a/pkg/front_end/lib/file_system.dart
+++ b/pkg/front_end/lib/file_system.dart
@@ -18,22 +18,17 @@ import 'package:path/path.dart' as path;
 /// Not intended to be implemented or extended by clients.
 abstract class FileSystem {
   /// Returns a path context suitable for use with this [FileSystem].
+  ///
+  /// TODO(paulberry): try to eliminate all usages of this.  Since the
+  /// FileSystem API now uses URIs rather than paths, it should not be needed.
   path.Context get context;
 
-  /// Returns a [FileSystemEntity] corresponding to the given [path].
-  ///
-  /// Uses of `..` and `.` in path are normalized before returning (so, for
-  /// example, `entityForPath('./foo')` and `entityForPath('foo')` are
-  /// equivalent).  Relative paths are also converted to absolute paths.
-  ///
-  /// Does not check whether a file or folder exists at the given location.
-  FileSystemEntity entityForPath(String path);
-
   /// Returns a [FileSystemEntity] corresponding to the given [uri].
   ///
   /// Uses of `..` and `.` in the URI are normalized before returning.
   ///
-  /// If [uri] is not an absolute `file:` URI, an [Error] will be thrown.
+  /// If the URI scheme is not supported by this file system, an [Error] will be
+  /// thrown.
   ///
   /// Does not check whether a file or folder exists at the given location.
   FileSystemEntity entityForUri(Uri uri);
@@ -46,14 +41,12 @@ abstract class FileSystem {
 ///
 /// Not intended to be implemented or extended by clients.
 abstract class FileSystemEntity {
-  /// Returns the absolute normalized path represented by this file system
+  /// Returns the absolute normalized URI represented by this file system
   /// entity.
   ///
-  /// Note: if the [FileSystemEntity] was created using
-  /// [FileSystem.entityForPath], this is not necessarily the same as the path
-  /// that was used to create the object, since the path might have been
-  /// normalized.
-  String get path;
+  /// Note: this is not necessarily the same as the URI that was passed to
+  /// [FileSystem.entityForUri], since the URI might have been normalized.
+  Uri get uri;
 
   /// Attempts to access this file system entity as a file and read its contents
   /// as raw bytes.
diff --git a/pkg/front_end/lib/kernel_generator.dart b/pkg/front_end/lib/kernel_generator.dart
index a87bb683b44..348d1f90215 100644
--- a/pkg/front_end/lib/kernel_generator.dart
+++ b/pkg/front_end/lib/kernel_generator.dart
@@ -59,7 +59,7 @@ Future<Program> kernelForProgram(Uri source, CompilerOptions options) async {
 /// which will be read are those listed in [sources],
 /// [CompilerOptions.inputSummaries], and [CompilerOptions.sdkSummary].  If a
 /// source file attempts to refer to a file which is not obtainable from these
-/// paths, that will result in an error, even if the file exists on the
+/// URIs, that will result in an error, even if the file exists on the
 /// filesystem.
 ///
 /// When [CompilerOptions.chaseDependencies] is true, this default behavior
@@ -128,7 +128,8 @@ Future<Program> kernelForBuildUnit(
 Future<DartLoader> _createLoader(CompilerOptions options,
     {Repository repository, Uri entry}) async {
   var kernelOptions = _convertOptions(options);
-  var packages = await createPackages(options.packagesFilePath,
+  var packages = await createPackages(
+      _uriToPath(options.packagesFileUri, options),
       discoveryPath: entry?.path);
   return new DartLoader(
       repository ?? new Repository(), kernelOptions, packages);
@@ -137,11 +138,12 @@ Future<DartLoader> _createLoader(CompilerOptions options,
 DartOptions _convertOptions(CompilerOptions options) {
   return new DartOptions(
       strongMode: options.strongMode,
-      sdk: options.sdkPath,
+      sdk: _uriToPath(options.sdkRoot, options),
       // TODO(sigmund): make it possible to use summaries and still compile the
       // sdk sources.
-      sdkSummary: options.compileSdk ? null : options.sdkSummary,
-      packagePath: options.packagesFilePath,
+      sdkSummary:
+          options.compileSdk ? null : _uriToPath(options.sdkSummary, options),
+      packagePath: _uriToPath(options.packagesFileUri, options),
       declaredVariables: options.declaredVariables);
 }
 
@@ -152,6 +154,14 @@ void _reportErrors(List errors, ErrorHandler onError) {
   }
 }
 
+String _uriToPath(Uri uri, CompilerOptions options) {
+  if (uri == null) return null;
+  if (uri.scheme != 'file') {
+    throw new StateError('Only file URIs are supported');
+  }
+  return options.fileSystem.context.fromUri(uri);
+}
+
 // TODO(sigmund): delete this class. Dartk should not format errors itself, we
 // should just pass them along.
 class _DartkError implements CompilationError {
diff --git a/pkg/front_end/lib/memory_file_system.dart b/pkg/front_end/lib/memory_file_system.dart
index 267cbe36a6a..e7382adfb38 100644
--- a/pkg/front_end/lib/memory_file_system.dart
+++ b/pkg/front_end/lib/memory_file_system.dart
@@ -20,26 +20,29 @@ class MemoryFileSystem implements FileSystem {
   @override
   final p.Context context;
 
-  final Map<String, Uint8List> _files = {};
+  final Map<Uri, Uint8List> _files = {};
 
   /// The "current directory" in the in-memory virtual file system.
   ///
-  /// This is used to convert relative paths to absolute paths.
-  String currentDirectory;
+  /// This is used to convert relative URIs to absolute URIs.
+  ///
+  /// Always ends in a trailing '/'.
+  Uri currentDirectory;
 
-  MemoryFileSystem(this.context, this.currentDirectory);
-
-  @override
-  MemoryFileSystemEntity entityForPath(String path) =>
-      new MemoryFileSystemEntity._(
-          this, context.normalize(context.join(currentDirectory, path)));
+  MemoryFileSystem(this.context, Uri currentDirectory)
+      : currentDirectory = _addTrailingSlash(currentDirectory);
 
   @override
   MemoryFileSystemEntity entityForUri(Uri uri) {
-    if (uri.scheme != 'file') throw new ArgumentError('File URI expected');
-    // Note: we don't have to verify that the URI's path is absolute, because
-    // URIs with non-empty schemes always have absolute paths.
-    return entityForPath(context.fromUri(uri));
+    return new MemoryFileSystemEntity._(
+        this, currentDirectory.resolveUri(uri).normalizePath());
+  }
+
+  static Uri _addTrailingSlash(Uri uri) {
+    if (!uri.path.endsWith('/')) {
+      uri = uri.replace(path: uri.path + '/');
+    }
+    return uri;
   }
 }
 
@@ -49,22 +52,22 @@ class MemoryFileSystemEntity implements FileSystemEntity {
   final MemoryFileSystem _fileSystem;
 
   @override
-  final String path;
+  final Uri uri;
 
-  MemoryFileSystemEntity._(this._fileSystem, this.path);
+  MemoryFileSystemEntity._(this._fileSystem, this.uri);
 
   @override
-  int get hashCode => path.hashCode;
+  int get hashCode => uri.hashCode;
 
   @override
   bool operator ==(Object other) =>
       other is MemoryFileSystemEntity &&
-      other.path == path &&
+      other.uri == uri &&
       identical(other._fileSystem, _fileSystem);
 
   @override
   Future<List<int>> readAsBytes() async {
-    List<int> contents = _fileSystem._files[path];
+    List<int> contents = _fileSystem._files[uri];
     if (contents != null) {
       return contents.toList();
     }
@@ -82,7 +85,7 @@ class MemoryFileSystemEntity implements FileSystemEntity {
   /// If no file exists, one is created.  If a file exists already, it is
   /// overwritten.
   void writeAsBytesSync(List<int> bytes) {
-    _fileSystem._files[path] = new Uint8List.fromList(bytes);
+    _fileSystem._files[uri] = new Uint8List.fromList(bytes);
   }
 
   /// Writes the given string to this file system entity.
@@ -95,6 +98,6 @@ class MemoryFileSystemEntity implements FileSystemEntity {
     // Note: the return type of UTF8.encode is List<int>, but in practice it
     // always returns Uint8List.  We rely on that for efficiency, so that we
     // don't have to make an extra copy.
-    _fileSystem._files[path] = UTF8.encode(s) as Uint8List;
+    _fileSystem._files[uri] = UTF8.encode(s) as Uint8List;
   }
 }
diff --git a/pkg/front_end/lib/physical_file_system.dart b/pkg/front_end/lib/physical_file_system.dart
index 0ed5d8b0926..757eb60e834 100644
--- a/pkg/front_end/lib/physical_file_system.dart
+++ b/pkg/front_end/lib/physical_file_system.dart
@@ -23,37 +23,39 @@ class PhysicalFileSystem implements FileSystem {
   @override
   p.Context get context => p.context;
 
-  @override
-  FileSystemEntity entityForPath(String path) =>
-      new _PhysicalFileSystemEntity(context.normalize(context.absolute(path)));
-
   @override
   FileSystemEntity entityForUri(Uri uri) {
-    if (uri.scheme != 'file') throw new ArgumentError('File URI expected');
+    if (uri.scheme != 'file' && uri.scheme != '') {
+      throw new ArgumentError('File URI expected');
+    }
     // Note: we don't have to verify that the URI's path is absolute, because
     // URIs with non-empty schemes always have absolute paths.
-    return entityForPath(context.fromUri(uri));
+    var path = context.fromUri(uri);
+    return new _PhysicalFileSystemEntity(
+        context.normalize(context.absolute(path)));
   }
 }
 
 /// Concrete implementation of [FileSystemEntity] for use by
 /// [PhysicalFileSystem].
 class _PhysicalFileSystemEntity implements FileSystemEntity {
-  @override
-  final String path;
+  final String _path;
 
-  _PhysicalFileSystemEntity(this.path);
+  _PhysicalFileSystemEntity(this._path);
 
   @override
-  int get hashCode => path.hashCode;
+  int get hashCode => _path.hashCode;
+
+  @override
+  Uri get uri => p.toUri(_path);
 
   @override
   bool operator ==(Object other) =>
-      other is _PhysicalFileSystemEntity && other.path == path;
+      other is _PhysicalFileSystemEntity && other._path == _path;
 
   @override
-  Future<List<int>> readAsBytes() => new io.File(path).readAsBytes();
+  Future<List<int>> readAsBytes() => new io.File(_path).readAsBytes();
 
   @override
-  Future<String> readAsString() => new io.File(path).readAsString();
+  Future<String> readAsString() => new io.File(_path).readAsString();
 }
diff --git a/pkg/front_end/lib/src/base/processed_options.dart b/pkg/front_end/lib/src/base/processed_options.dart
index e1a4ef4dcba..360b5fa6a48 100644
--- a/pkg/front_end/lib/src/base/processed_options.dart
+++ b/pkg/front_end/lib/src/base/processed_options.dart
@@ -39,11 +39,11 @@ class ProcessedOptions {
   /// Get the [FileSystem] which should be used by the front end to access
   /// files.
   ///
-  /// If the client supplied bazel roots using [CompilerOptions.bazelRoots], the
+  /// If the client supplied roots using [CompilerOptions.multiRoots], the
   /// returned [FileSystem] will automatically perform the appropriate mapping.
   FileSystem get fileSystem {
-    // TODO(paulberry): support bazelRoots.
-    assert(_raw.bazelRoots.isEmpty);
+    // TODO(paulberry): support multiRoots.
+    assert(_raw.multiRoots.isEmpty);
     return _raw.fileSystem;
   }
 
@@ -56,8 +56,7 @@ class ProcessedOptions {
       await _getPackages();
       var sdkLibraries =
           <String, Uri>{}; // TODO(paulberry): support SDK libraries
-      _uriResolver =
-          new UriResolver(_packages, sdkLibraries, fileSystem.context);
+      _uriResolver = new UriResolver(_packages, sdkLibraries);
     }
     return _uriResolver;
   }
@@ -68,15 +67,14 @@ class ProcessedOptions {
   /// required to locate/read the packages file.
   Future<Map<String, Uri>> _getPackages() async {
     if (_packages == null) {
-      if (_raw.packagesFilePath == null) {
+      if (_raw.packagesFileUri == null) {
         throw new UnimplementedError(); // TODO(paulberry): search for .packages
-      } else if (_raw.packagesFilePath.isEmpty) {
+      } else if (_raw.packagesFileUri.path.isEmpty) {
         _packages = {};
       } else {
         var contents =
-            await fileSystem.entityForPath(_raw.packagesFilePath).readAsBytes();
-        var baseLocation = fileSystem.context.toUri(_raw.packagesFilePath);
-        _packages = package_config.parse(contents, baseLocation);
+            await fileSystem.entityForUri(_raw.packagesFileUri).readAsBytes();
+        _packages = package_config.parse(contents, _raw.packagesFileUri);
       }
     }
     return _packages;
diff --git a/pkg/front_end/lib/src/base/uri_resolver.dart b/pkg/front_end/lib/src/base/uri_resolver.dart
index db469f0003d..0c6a4209743 100644
--- a/pkg/front_end/lib/src/base/uri_resolver.dart
+++ b/pkg/front_end/lib/src/base/uri_resolver.dart
@@ -2,13 +2,15 @@
 // 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.
 
-import 'package:path/path.dart' as p;
-
-/// The class `UriResolver` implements the rules for resolving URIs to file
-/// paths.
-///
-/// TODO(paulberry): Is it necessary to support the "http" scheme?
+/// The class `UriResolver` implements the rules for resolving "dart:" and
+/// "package:" URIs.
 class UriResolver {
+  /// The URI scheme used for "package" URIs.
+  static const PACKAGE_SCHEME = 'package';
+
+  /// The URI scheme used for "dart" URIs.
+  static const DART_SCHEME = 'dart';
+
   /// A map from package name to the file URI of the "lib" directory of the
   /// corresponding package.  This is equivalent to the format returned by
   /// the "package_config" package's parse() function.
@@ -18,36 +20,27 @@ class UriResolver {
   /// of the defining compilation unit of the SDK library.
   final Map<String, Uri> sdkLibraries;
 
-  /// The path context which should be used to convert from file URIs to file
-  /// paths.
-  final p.Context pathContext;
+  UriResolver(this.packages, this.sdkLibraries);
 
-  /// The URI scheme used for "package" URIs.
-  static const PACKAGE_SCHEME = 'package';
-
-  /// The URI scheme used for "dart" URIs.
-  static const DART_SCHEME = 'dart';
-
-  /// The URI scheme used for "file" URIs.
-  static const FILE_SCHEME = 'file';
-
-  UriResolver(this.packages, this.sdkLibraries, this.pathContext);
-
-  /// Converts a URI to a file path.
+  /// Converts "package:" and "dart:" URIs to the locations of the corresponding
+  /// files.
   ///
-  /// If the given URI is valid, and of a recognized form, returns the file path
-  /// it corresponds to.  Otherwise returns `null`.  It is not necessary for the
-  /// URI to be absolute (relative URIs will be converted to relative file
-  /// paths).
+  /// If the given URI is a "package:" or "dart:" URI, is well formed, and names
+  /// a package or dart library that is recognized, returns the URI it resolves
+  /// to.  If the given URI is a "package:" or "dart:" URI, and is ill-formed
+  /// or names a package or dart library that is not recognized, returns `null`.
   ///
-  /// Note that no I/O is performed; the file path that is returned will be
+  /// If the given URI has any scheme other than "package:" or "dart:", it is
+  /// returned unchanged.
+  ///
+  /// It is not necessary for the URI to be absolute (relative URIs will be
+  /// passed through unchanged).
+  ///
+  /// Note that no I/O is performed; the URI that is returned will be
   /// independent of whether or not any particular file exists on the file
   /// system.
-  String resolve(Uri uri) {
-    Uri fileUri;
-    if (uri.scheme == FILE_SCHEME) {
-      fileUri = uri;
-    } else {
+  Uri resolve(Uri uri) {
+    if (uri.scheme == DART_SCHEME || uri.scheme == PACKAGE_SCHEME) {
       var path = uri.path;
       var slashIndex = path.indexOf('/');
       String prefix;
@@ -67,9 +60,9 @@ class UriResolver {
         libUri = sdkLibraries[prefix];
       }
       if (libUri == null) return null;
-      fileUri = libUri.resolve(rest);
-      if (fileUri.scheme != FILE_SCHEME) return null;
+      return libUri.resolve(rest);
+    } else {
+      return uri;
     }
-    return pathContext.fromUri(fileUri);
   }
 }
diff --git a/pkg/front_end/lib/src/dependency_grapher_impl.dart b/pkg/front_end/lib/src/dependency_grapher_impl.dart
index 5dbe12d998d..09519315674 100644
--- a/pkg/front_end/lib/src/dependency_grapher_impl.dart
+++ b/pkg/front_end/lib/src/dependency_grapher_impl.dart
@@ -99,13 +99,14 @@ class _WalkerNode extends Node<_WalkerNode> {
   Future<List<_WalkerNode>> computeDependencies() async {
     var dependencies = <_WalkerNode>[];
     // TODO(paulberry): add error recovery if the file can't be read.
-    var path = walker.uriResolver.resolve(uri);
-    if (path == null) {
+    var resolvedUri = walker.uriResolver.resolve(uri);
+    if (resolvedUri == null) {
       // TODO(paulberry): If an error reporter was provided, report the error
       // in the proper way and continue.
       throw new StateError('Invalid URI: $uri');
     }
-    var contents = await walker.fileSystem.entityForPath(path).readAsString();
+    var contents =
+        await walker.fileSystem.entityForUri(resolvedUri).readAsString();
     var scanner = new _Scanner(contents);
     var token = scanner.tokenize();
     // TODO(paulberry): report errors.
diff --git a/pkg/front_end/test/dependency_grapher_test.dart b/pkg/front_end/test/dependency_grapher_test.dart
index bcfca56ae25..826283d9d81 100644
--- a/pkg/front_end/test/dependency_grapher_test.dart
+++ b/pkg/front_end/test/dependency_grapher_test.dart
@@ -35,15 +35,17 @@ class DependencyGrapherTest {
     // If no starting points given, assume the first entry in [contents] is the
     // single starting point.
     startingPoints ??= [contents.keys.first];
-    var fileSystem = new MemoryFileSystem(pathos.posix, '/');
+    var fileSystem = new MemoryFileSystem(pathos.posix, Uri.parse('file:///'));
     contents.forEach((path, text) {
-      fileSystem.entityForPath(path).writeAsStringSync(text);
+      fileSystem.entityForUri(pathos.posix.toUri(path)).writeAsStringSync(text);
     });
     // TODO(paulberry): implement and test other option possibilities.
     var options = new CompilerOptions()
       ..fileSystem = fileSystem
       ..chaseDependencies = true
-      ..packagesFilePath = packagesFilePath;
+      ..packagesFileUri = packagesFilePath == ''
+          ? new Uri()
+          : pathos.posix.toUri(packagesFilePath);
     var graph = await graphForProgram(
         startingPoints.map(pathos.posix.toUri).toList(), options);
     return graph.topologicallySortedCycles;
diff --git a/pkg/front_end/test/memory_file_system_test.dart b/pkg/front_end/test/memory_file_system_test.dart
index b9757f3926a..365cd43af76 100644
--- a/pkg/front_end/test/memory_file_system_test.dart
+++ b/pkg/front_end/test/memory_file_system_test.dart
@@ -30,26 +30,23 @@ class FileTest extends _BaseTestNative {
   setUp() {
     super.setUp();
     path = join(tempPath, 'file.txt');
-    file = fileSystem.entityForPath(path);
+    file = entityForPath(path);
   }
 
   test_equals_differentPaths() {
-    expect(
-        file == fileSystem.entityForPath(join(tempPath, 'file2.txt')), isFalse);
+    expect(file == entityForPath(join(tempPath, 'file2.txt')), isFalse);
   }
 
   test_equals_samePath() {
-    expect(
-        file == fileSystem.entityForPath(join(tempPath, 'file.txt')), isTrue);
+    expect(file == entityForPath(join(tempPath, 'file.txt')), isTrue);
   }
 
   test_hashCode_samePath() {
-    expect(file.hashCode,
-        fileSystem.entityForPath(join(tempPath, 'file.txt')).hashCode);
+    expect(file.hashCode, entityForPath(join(tempPath, 'file.txt')).hashCode);
   }
 
   test_path() {
-    expect(file.path, path);
+    expect(file.uri, fileSystem.context.toUri(path));
   }
 
   test_readAsBytes_badUtf8() async {
@@ -128,40 +125,44 @@ abstract class MemoryFileSystemTestMixin extends _BaseTest {
     tempUri = fileSystem.context.toUri(tempPath);
   }
 
-  test_entityForPath() {
-    var path = join(tempPath, 'file.txt');
-    expect(fileSystem.entityForPath(path).path, path);
+  test_currentDirectory_trailingSlash() {
+    // The currentDirectory should already end in a trailing slash.
+    expect(fileSystem.currentDirectory.path, endsWith('/'));
+    // A trailing slash should automatically be appended when creating a
+    // MemoryFileSystem.
+    var path = fileSystem.currentDirectory.path;
+    var currentDirectoryWithoutSlash = fileSystem.currentDirectory
+        .replace(path: path.substring(0, path.length - 1));
+    expect(
+        new MemoryFileSystem(fileSystem.context, currentDirectoryWithoutSlash)
+            .currentDirectory,
+        fileSystem.currentDirectory);
+    // If the currentDirectory supplied to the MemoryFileSystem constructor
+    // already has a trailing slash, no further trailing slash should be added.
+    expect(
+        new MemoryFileSystem(fileSystem.context, fileSystem.currentDirectory)
+            .currentDirectory,
+        fileSystem.currentDirectory);
   }
 
   test_entityForPath_absolutize() {
-    expect(fileSystem.entityForPath('file.txt').path,
-        join(fileSystem.currentDirectory, 'file.txt'));
+    expect(entityForPath('file.txt').uri,
+        fileSystem.currentDirectory.resolve('file.txt'));
   }
 
   test_entityForPath_normalize_dot() {
-    expect(fileSystem.entityForPath(join(tempPath, '.', 'file.txt')).path,
-        join(tempPath, 'file.txt'));
+    expect(entityForPath(join(tempPath, '.', 'file.txt')).uri,
+        Uri.parse('$tempUri/file.txt'));
   }
 
   test_entityForPath_normalize_dotDot() {
-    expect(
-        fileSystem.entityForPath(join(tempPath, 'foo', '..', 'file.txt')).path,
-        join(tempPath, 'file.txt'));
+    expect(entityForPath(join(tempPath, 'foo', '..', 'file.txt')).uri,
+        Uri.parse('$tempUri/file.txt'));
   }
 
   test_entityForUri() {
-    expect(fileSystem.entityForUri(Uri.parse('$tempUri/file.txt')).path,
-        join(tempPath, 'file.txt'));
-  }
-
-  test_entityForUri_bareUri_absolute() {
-    expect(() => fileSystem.entityForUri(Uri.parse('/file.txt')),
-        throwsA(new isInstanceOf<Error>()));
-  }
-
-  test_entityForUri_bareUri_relative() {
-    expect(() => fileSystem.entityForUri(Uri.parse('file.txt')),
-        throwsA(new isInstanceOf<Error>()));
+    expect(fileSystem.entityForUri(Uri.parse('$tempUri/file.txt')).uri,
+        Uri.parse('$tempUri/file.txt'));
   }
 
   test_entityForUri_fileUri_relative() {
@@ -184,18 +185,18 @@ abstract class MemoryFileSystemTestMixin extends _BaseTest {
   }
 
   test_entityForUri_nonFileUri() {
-    expect(() => fileSystem.entityForUri(Uri.parse('package:foo/bar.dart')),
-        throwsA(new isInstanceOf<Error>()));
+    var uri = Uri.parse('package:foo/bar.dart');
+    expect(fileSystem.entityForUri(uri).uri, uri);
   }
 
   test_entityForUri_normalize_dot() {
-    expect(fileSystem.entityForUri(Uri.parse('$tempUri/./file.txt')).path,
-        join(tempPath, 'file.txt'));
+    expect(fileSystem.entityForUri(Uri.parse('$tempUri/./file.txt')).uri,
+        Uri.parse('$tempUri/file.txt'));
   }
 
   test_entityForUri_normalize_dotDot() {
-    expect(fileSystem.entityForUri(Uri.parse('$tempUri/foo/../file.txt')).path,
-        join(tempPath, 'file.txt'));
+    expect(fileSystem.entityForUri(Uri.parse('$tempUri/foo/../file.txt')).uri,
+        Uri.parse('$tempUri/file.txt'));
   }
 }
 
@@ -213,8 +214,14 @@ class MemoryFileSystemTestWindows extends _BaseTestWindows
 
 abstract class _BaseTest {
   MemoryFileSystem get fileSystem;
+
   String get tempPath;
+
+  MemoryFileSystemEntity entityForPath(String path) =>
+      fileSystem.entityForUri(fileSystem.context.toUri(path));
+
   String join(String path1, String path2, [String path3, String path4]);
+
   void setUp();
 }
 
@@ -227,8 +234,8 @@ class _BaseTestNative extends _BaseTest {
 
   setUp() {
     tempPath = pathos.join(io.Directory.systemTemp.path, 'test_file_system');
-    fileSystem =
-        new MemoryFileSystem(pathos.context, io.Directory.current.path);
+    fileSystem = new MemoryFileSystem(
+        pathos.context, pathos.toUri(io.Directory.current.path));
   }
 }
 
@@ -241,7 +248,7 @@ class _BaseTestPosix extends _BaseTest {
 
   void setUp() {
     tempPath = '/test_file_system';
-    fileSystem = new MemoryFileSystem(pathos.posix, '/cwd');
+    fileSystem = new MemoryFileSystem(pathos.posix, Uri.parse('file:///cwd'));
   }
 }
 
@@ -254,6 +261,7 @@ class _BaseTestWindows extends _BaseTest {
 
   void setUp() {
     tempPath = r'c:\test_file_system';
-    fileSystem = new MemoryFileSystem(pathos.windows, r'c:\cwd');
+    fileSystem =
+        new MemoryFileSystem(pathos.windows, Uri.parse('file:///c:/cwd'));
   }
 }
diff --git a/pkg/front_end/test/physical_file_system_test.dart b/pkg/front_end/test/physical_file_system_test.dart
index 859e998a58f..2a8cbb6377b 100644
--- a/pkg/front_end/test/physical_file_system_test.dart
+++ b/pkg/front_end/test/physical_file_system_test.dart
@@ -30,35 +30,19 @@ class FileTest extends _BaseTest {
   setUp() {
     super.setUp();
     path = p.join(tempPath, 'file.txt');
-    file = PhysicalFileSystem.instance.entityForPath(path);
+    file = PhysicalFileSystem.instance.entityForUri(p.toUri(path));
   }
 
   test_equals_differentPaths() {
-    expect(
-        file ==
-            PhysicalFileSystem.instance
-                .entityForPath(p.join(tempPath, 'file2.txt')),
-        isFalse);
+    expect(file == entityForPath(p.join(tempPath, 'file2.txt')), isFalse);
   }
 
   test_equals_samePath() {
-    expect(
-        file ==
-            PhysicalFileSystem.instance
-                .entityForPath(p.join(tempPath, 'file.txt')),
-        isTrue);
+    expect(file == entityForPath(p.join(tempPath, 'file.txt')), isTrue);
   }
 
   test_hashCode_samePath() {
-    expect(
-        file.hashCode,
-        PhysicalFileSystem.instance
-            .entityForPath(p.join(tempPath, 'file.txt'))
-            .hashCode);
-  }
-
-  test_path() {
-    expect(file.path, path);
+    expect(file.hashCode, entityForPath(p.join(tempPath, 'file.txt')).hashCode);
   }
 
   test_readAsBytes_badUtf8() async {
@@ -98,6 +82,10 @@ class FileTest extends _BaseTest {
     new io.File(path).writeAsBytesSync(bytes);
     expect(await file.readAsString(), '\u20ac');
   }
+
+  test_uri() {
+    expect(file.uri, p.toUri(path));
+  }
 }
 
 @reflectiveTest
@@ -111,48 +99,35 @@ class PhysicalFileSystemTest extends _BaseTest {
 
   test_entityForPath() {
     var path = p.join(tempPath, 'file.txt');
-    expect(PhysicalFileSystem.instance.entityForPath(path).path, path);
+    expect(entityForPath(path).uri, p.toUri(path));
   }
 
   test_entityForPath_absolutize() {
-    expect(PhysicalFileSystem.instance.entityForPath('file.txt').path,
-        new io.File('file.txt').absolute.path);
+    expect(entityForPath('file.txt').uri,
+        p.toUri(new io.File('file.txt').absolute.path));
   }
 
   test_entityForPath_normalize_dot() {
-    expect(
-        PhysicalFileSystem.instance
-            .entityForPath(p.join(tempPath, '.', 'file.txt'))
-            .path,
-        p.join(tempPath, 'file.txt'));
+    expect(entityForPath(p.join(tempPath, '.', 'file.txt')).uri,
+        p.toUri(p.join(tempPath, 'file.txt')));
   }
 
   test_entityForPath_normalize_dotDot() {
-    expect(
-        PhysicalFileSystem.instance
-            .entityForPath(p.join(tempPath, 'foo', '..', 'file.txt'))
-            .path,
-        p.join(tempPath, 'file.txt'));
+    expect(entityForPath(p.join(tempPath, 'foo', '..', 'file.txt')).uri,
+        p.toUri(p.join(tempPath, 'file.txt')));
   }
 
   test_entityForUri() {
     expect(
         PhysicalFileSystem.instance
             .entityForUri(Uri.parse('$tempUri/file.txt'))
-            .path,
-        p.join(tempPath, 'file.txt'));
+            .uri,
+        p.toUri(p.join(tempPath, 'file.txt')));
   }
 
   test_entityForUri_bareUri_absolute() {
-    expect(
-        () => PhysicalFileSystem.instance.entityForUri(Uri.parse('/file.txt')),
-        throwsA(new isInstanceOf<Error>()));
-  }
-
-  test_entityForUri_bareUri_relative() {
-    expect(
-        () => PhysicalFileSystem.instance.entityForUri(Uri.parse('file.txt')),
-        throwsA(new isInstanceOf<Error>()));
+    expect(PhysicalFileSystem.instance.entityForUri(Uri.parse('/file.txt')).uri,
+        p.toUri(p.fromUri('/file.txt')));
   }
 
   test_entityForUri_fileUri_relative() {
@@ -185,16 +160,16 @@ class PhysicalFileSystemTest extends _BaseTest {
     expect(
         PhysicalFileSystem.instance
             .entityForUri(Uri.parse('$tempUri/./file.txt'))
-            .path,
-        p.join(tempPath, 'file.txt'));
+            .uri,
+        p.toUri(p.join(tempPath, 'file.txt')));
   }
 
   test_entityForUri_normalize_dotDot() {
     expect(
         PhysicalFileSystem.instance
             .entityForUri(Uri.parse('$tempUri/foo/../file.txt'))
-            .path,
-        p.join(tempPath, 'file.txt'));
+            .uri,
+        p.toUri(p.join(tempPath, 'file.txt')));
   }
 }
 
@@ -202,6 +177,9 @@ class _BaseTest {
   io.Directory tempDirectory;
   String tempPath;
 
+  FileSystemEntity entityForPath(String path) =>
+      PhysicalFileSystem.instance.entityForUri(p.toUri(path));
+
   setUp() {
     tempDirectory = io.Directory.systemTemp.createTempSync('test_file_system');
     tempPath = tempDirectory.absolute.path;
diff --git a/pkg/front_end/test/src/base/processed_options_test.dart b/pkg/front_end/test/src/base/processed_options_test.dart
index ff34028afb3..76151dd0339 100644
--- a/pkg/front_end/test/src/base/processed_options_test.dart
+++ b/pkg/front_end/test/src/base/processed_options_test.dart
@@ -17,7 +17,7 @@ main() {
 
 @reflectiveTest
 class ProcessedOptionsTest {
-  final fileSystem = new MemoryFileSystem(pathos.posix, '/');
+  final fileSystem = new MemoryFileSystem(pathos.posix, Uri.parse('file:///'));
 
   test_compileSdk_false() {
     for (var value in [false, true]) {
@@ -37,46 +37,49 @@ class ProcessedOptionsTest {
 
   test_getUriResolver_explicitPackagesFile() async {
     // This .packages file should be ignored.
-    fileSystem.entityForPath('/.packages').writeAsStringSync('foo:bar\n');
+    fileSystem
+        .entityForUri(Uri.parse('file:///.packages'))
+        .writeAsStringSync('foo:bar\n');
     // This one should be used.
     fileSystem
-        .entityForPath('/explicit.packages')
+        .entityForUri(Uri.parse('file:///explicit.packages'))
         .writeAsStringSync('foo:baz\n');
     var raw = new CompilerOptions()
       ..fileSystem = fileSystem
-      ..packagesFilePath = '/explicit.packages';
+      ..packagesFileUri = Uri.parse('file:///explicit.packages');
     var processed = new ProcessedOptions(raw);
     var uriResolver = await processed.getUriResolver();
     expect(uriResolver.packages, {'foo': Uri.parse('file:///baz/')});
-    expect(uriResolver.pathContext, same(fileSystem.context));
   }
 
   test_getUriResolver_explicitPackagesFile_withBaseLocation() async {
     // This .packages file should be ignored.
-    fileSystem.entityForPath('/.packages').writeAsStringSync('foo:bar\n');
+    fileSystem
+        .entityForUri(Uri.parse('file:///.packages'))
+        .writeAsStringSync('foo:bar\n');
     // This one should be used.
     fileSystem
-        .entityForPath('/base/location/explicit.packages')
+        .entityForUri(Uri.parse('file:///base/location/explicit.packages'))
         .writeAsStringSync('foo:baz\n');
     var raw = new CompilerOptions()
       ..fileSystem = fileSystem
-      ..packagesFilePath = '/base/location/explicit.packages';
+      ..packagesFileUri = Uri.parse('file:///base/location/explicit.packages');
     var processed = new ProcessedOptions(raw);
     var uriResolver = await processed.getUriResolver();
     expect(
         uriResolver.packages, {'foo': Uri.parse('file:///base/location/baz/')});
-    expect(uriResolver.pathContext, same(fileSystem.context));
   }
 
   test_getUriResolver_noPackages() async {
     // .packages file should be ignored.
-    fileSystem.entityForPath('/.packages').writeAsStringSync('foo:bar\n');
+    fileSystem
+        .entityForUri(Uri.parse('file:///.packages'))
+        .writeAsStringSync('foo:bar\n');
     var raw = new CompilerOptions()
       ..fileSystem = fileSystem
-      ..packagesFilePath = '';
+      ..packagesFileUri = new Uri();
     var processed = new ProcessedOptions(raw);
     var uriResolver = await processed.getUriResolver();
     expect(uriResolver.packages, isEmpty);
-    expect(uriResolver.pathContext, same(fileSystem.context));
   }
 }
diff --git a/pkg/front_end/test/src/base/uri_resolver_test.dart b/pkg/front_end/test/src/base/uri_resolver_test.dart
index 1273a193448..af00240f74d 100644
--- a/pkg/front_end/test/src/base/uri_resolver_test.dart
+++ b/pkg/front_end/test/src/base/uri_resolver_test.dart
@@ -21,7 +21,7 @@ abstract class UriResolverTest {
   p.Context get pathContext;
 
   void test_badScheme() {
-    _expectResolution('foo:bar/baz.dart', null);
+    _expectResolutionUri('foo:bar/baz.dart', Uri.parse('foo:bar/baz.dart'));
   }
 
   void test_dart() {
@@ -54,11 +54,11 @@ abstract class UriResolverTest {
   }
 
   void test_noSchemeAbsolute() {
-    _expectResolution('/foo.dart', null);
+    _expectResolutionUri('/foo.dart', Uri.parse('/foo.dart'));
   }
 
   void test_noSchemeRelative() {
-    _expectResolution('foo.dart', null);
+    _expectResolution('foo.dart', 'foo.dart');
   }
 
   void test_package() {
@@ -102,6 +102,13 @@ abstract class UriResolverTest {
   /// Verifies that the resolution of [uriString] produces the path
   /// [expectedResult].
   void _expectResolution(String uriString, String expectedResult) {
+    _expectResolutionUri(uriString,
+        expectedResult == null ? null : pathContext.toUri(expectedResult));
+  }
+
+  /// Verifies that the resolution of [uriString] produces the URI
+  /// [expectedResult].
+  void _expectResolutionUri(String uriString, Uri expectedResult) {
     var packages = {
       'foo': _u('packages/foo/lib/'),
       'bar': _u('packages/bar/lib/')
@@ -110,7 +117,7 @@ abstract class UriResolverTest {
       'core': _u('sdk/lib/core/core.dart'),
       'async': _u('sdk/lib/async/async.dart')
     };
-    var uriResolver = new UriResolver(packages, sdkLibraries, pathContext);
+    var uriResolver = new UriResolver(packages, sdkLibraries);
     expect(uriResolver.resolve(Uri.parse(uriString)), expectedResult);
   }
 
diff --git a/pkg/front_end/tool/example.dart b/pkg/front_end/tool/example.dart
index 1d49b228759..d81f43746e3 100644
--- a/pkg/front_end/tool/example.dart
+++ b/pkg/front_end/tool/example.dart
@@ -11,17 +11,19 @@ Future dumpToSink(Program program, StreamSink<List<int>> sink) {
 }
 
 Future kernelToSink(Uri entry, StreamSink<List<int>> sink) async {
-  var program = await kernelForProgram(entry,
+  var program = await kernelForProgram(
+      entry,
       new CompilerOptions()
-        ..sdkPath = 'sdk'
-        ..packagesFilePath = '.packages'
+        ..sdkRoot = new Uri.file('sdk')
+        ..packagesFileUri = new Uri.file('.packages')
         ..onError = (e) => print(e.message));
 
   await dumpToSink(program, sink);
 }
 
 main(args) async {
-  kernelToSink(Uri.base.resolve(args[0]),
+  kernelToSink(
+      Uri.base.resolve(args[0]),
       // TODO(sigmund,hausner): define memory type where to dump binary data.
       new StreamController<List<int>>.broadcast().sink);
 }
diff --git a/pkg/front_end/tool/perf.dart b/pkg/front_end/tool/perf.dart
index a69251bb818..7c4e4cd1d7b 100644
--- a/pkg/front_end/tool/perf.dart
+++ b/pkg/front_end/tool/perf.dart
@@ -115,15 +115,16 @@ Future<Program> generateKernel(Uri entryUri,
   var options = new CompilerOptions()
     ..strongMode = false
     ..compileSdk = compileSdk
-    ..packagesFilePath = '.packages'
+    ..packagesFileUri = new Uri.file('.packages')
     ..onError = ((e) => print('${e.message}'));
   if (useSdkSummary) {
     // TODO(sigmund): adjust path based on the benchmark runner architecture.
     // Possibly let the runner make the file available at an architecture
     // independent location.
-    options.sdkSummary = 'out/ReleaseX64/dart-sdk/lib/_internal/spec.sum';
+    options.sdkSummary =
+        new Uri.file('out/ReleaseX64/dart-sdk/lib/_internal/spec.sum');
   } else {
-    options.sdkPath = 'sdk';
+    options.sdkRoot = new Uri.file('sdk');
   }
   Program program = await kernelForProgram(entryUri, options);
   dartkTimer.stop();