Fix for resolving 'bar.dart' against 'dart:foo'. Test for TranslateUri.

pkg/front_end/test/fasta/translate_uri_test.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.
+
+import 'package:front_end/src/fasta/translate_uri.dart';
+import 'package:test/test.dart';
+import 'package:test_reflective_loader/test_reflective_loader.dart';
+
+main() {
+  defineReflectiveSuite(() {
+    defineReflectiveTests(TranslateUriTest);
+  });
+}
+
+@reflectiveTest
+class TranslateUriTest {
+  void test_translate_dart() {
+    var translator = new TranslateUri({}, {
+      'core': Uri.parse('file:///sdk/core/core.dart'),
+      'math': Uri.parse('file:///sdk/math/math.dart')
+    });
+
+    expect(translator.translate(Uri.parse('dart:core')),
+        Uri.parse('file:///sdk/core/core.dart'));
+    expect(translator.translate(Uri.parse('dart:core/string.dart')),
+        Uri.parse('file:///sdk/core/string.dart'));
+
+    expect(translator.translate(Uri.parse('dart:math')),
+        Uri.parse('file:///sdk/math/math.dart'));
+
+    expect(translator.translate(Uri.parse('dart:unknown')), isNull);
+  }
+}