Implement URI resolution in the front end.

pkg/front_end/lib/src/base/uri_resolver.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.
+
+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?
+class UriResolver {
+  /// 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.
+  final Map<String, Uri> packages;
+
+  /// A map from SDK library name (e.g. `core` for `dart:core`) to the file URI
+  /// 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;
+
+  /// 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.
+  ///
+  /// 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).
+  ///
+  /// Note that no I/O is performed; the file path 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 {
+      var path = uri.path;
+      var slashIndex = path.indexOf('/');
+      String prefix;
+      String rest;
+      if (slashIndex >= 0) {
+        prefix = path.substring(0, slashIndex);
+        rest = path.substring(slashIndex + 1);
+      } else {
+        prefix = path;
+        rest = '';
+      }
+      Uri libUri;
+      if (uri.scheme == PACKAGE_SCHEME) {
+        if (slashIndex < 0) return null;
+        libUri = packages[prefix];
+      } else if (uri.scheme == DART_SCHEME) {
+        libUri = sdkLibraries[prefix];
+      }
+      if (libUri == null) return null;
+      fileUri = libUri.resolve(rest);
+      if (fileUri.scheme != FILE_SCHEME) return null;
+    }
+    return pathContext.fromUri(fileUri);
+  }
+}