fixes #201, add support for custom URL mapping

lib/src/testing.dart

Index: lib/src/testing.dart
diff --git a/lib/src/testing.dart b/lib/src/testing.dart
index e8807e77e2745a72104c830bdc8591b606a73dc8..b514170ac869184f45a78ba82916a3a520c2b11b 100644
--- a/lib/src/testing.dart
+++ b/lib/src/testing.dart
@@ -54,7 +54,8 @@ CheckerResults testChecker(Map<String, String> testFiles,
     inferFromOverrides: ResolverOptions.inferFromOverridesDefault,
     inferTransitively: ResolverOptions.inferTransitivelyDefault,
     nonnullableTypes: TypeOptions.NONNULLABLE_TYPES,
-    wrapClosures: RulesOptions.wrapClosuresDefault}) {
+    wrapClosures: RulesOptions.wrapClosuresDefault,
+    customUrlMappings: const {}}) {
   expect(testFiles.containsKey('/main.dart'), isTrue,
       reason: '`/main.dart` is missing in testFiles');
 
@@ -73,7 +74,8 @@ CheckerResults testChecker(Map<String, String> testFiles,
       useMockSdk: sdkDir == null,
       dartSdkPath: sdkDir,
       runtimeDir: '/dev_compiler_runtime/',
-      entryPointFile: '/main.dart');
+      entryPointFile: '/main.dart',
+      customUrlMappings: customUrlMappings);
 
   var context = createAnalysisContext(options, fileResolvers: [uriResolver]);