Return empty contents for not existing sources.

pkg/analyzer/test/src/context/context_test.dart

Index: pkg/analyzer/test/src/context/context_test.dart
diff --git a/pkg/analyzer/test/src/context/context_test.dart b/pkg/analyzer/test/src/context/context_test.dart
index 24899224d66136f098644538729e22aa42780a63..d7e9986895f79b5f481a340e7b11b8f22a5b1f68 100644
--- a/pkg/analyzer/test/src/context/context_test.dart
+++ b/pkg/analyzer/test/src/context/context_test.dart
@@ -682,7 +682,7 @@ void g() { f(null); }''');
     expect(error.errorCode, ScannerErrorCode.UNABLE_GET_CONTENT);
   }
 
-  void fail_performAnalysisTask_importedLibraryAdd() {
+  void test_performAnalysisTask_importedLibraryAdd() {
     Source libASource =
         addSource("/libA.dart", "library libA; import 'libB.dart';");
     _analyzeAll_assertFinished();
@@ -700,8 +700,8 @@ void g() { f(null); }''');
     expect(
         context.getResolvedCompilationUnit2(libBSource, libBSource), isNotNull,
         reason: "libB resolved 2");
-    expect(!_hasAnalysisErrorWithErrorSeverity(context.getErrors(libASource)),
-        isTrue, reason: "libA doesn't have errors");
+    expect(_hasAnalysisErrorWithErrorSeverity(context.getErrors(libASource)),
+        isFalse, reason: "libA doesn't have errors");
   }
 
   void fail_performAnalysisTask_importedLibraryAdd_html() {