Move resolveRelativeUri into front_end.

pkg/front_end/lib/src/base/resolve_relative_uri.dart

+// Copyright (c) 2017, 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.
+
+/**
+ * Resolve the [containedUri] against [baseUri] using Dart rules.
+ *
+ * This function behaves similarly to [Uri.resolveUri], except that it properly
+ * handles situations like the following:
+ *
+ *     resolveRelativeUri(dart:core, bool.dart) -> dart:core/bool.dart
+ *     resolveRelativeUri(package:a/b.dart, ../c.dart) -> package:a/c.dart
+ */
+Uri resolveRelativeUri(Uri baseUri, Uri containedUri) {
+  if (containedUri.isAbsolute) {
+    return containedUri;
+  }
+  String scheme = baseUri.scheme;
+  // dart:core => dart:core/core.dart
+  if (scheme == 'dart') {
+    String part = baseUri.path;
+    if (part.indexOf('/') < 0) {
+      baseUri = Uri.parse('$scheme:$part/$part.dart');
+    }
+  }
+  return baseUri.resolveUri(containedUri);
+}