Refactorings for TranslateUri and its tests.

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({}, {
+  void test_isPlatformImplementation() {
+    var translator = new TranslateUri({
       'core': Uri.parse('file:///sdk/core/core.dart'),
       'math': Uri.parse('file:///sdk/math/math.dart')
-    }, {});
+    }, {}, {});
+
+    bool isPlatform(String uriStr) {
+      var uri = Uri.parse(uriStr);
+      return translator.isPlatformImplementation(uri);
+    }
+
+    expect(isPlatform('dart:core/string.dart'), isTrue);
+    expect(isPlatform('dart:core'), isFalse);
+    expect(isPlatform('dart:_builtin'), isTrue);
+    expect(isPlatform('file:///sdk/math/math.dart'), isFalse);
+  }
+
+  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);
-
-    expect(
-        translator.isPlatformImplementation(Uri.parse('dart:core/string.dart')),
-        isTrue);
-    expect(
-        translator.isPlatformImplementation(Uri.parse('dart:core')), isFalse);
-    expect(translator.isPlatformImplementation(Uri.parse('dart:_builtin')),
-        isTrue);
-    expect(
-        translator
-            .isPlatformImplementation(Uri.parse('file:///sdk/math/math.dart')),
-        isFalse);
   }
 }