Create a NullErrorReporter to use when errors don't need to be recorded.

pkg/analyzer/lib/src/generated/engine.dart

Index: pkg/analyzer/lib/src/generated/engine.dart
diff --git a/pkg/analyzer/lib/src/generated/engine.dart b/pkg/analyzer/lib/src/generated/engine.dart
index a37d3cbb6baed9bbe62f422756d6e1fa2205d5a1..f7a081eb87021759e0e91106f4607c73d173f880 100644
--- a/pkg/analyzer/lib/src/generated/engine.dart
+++ b/pkg/analyzer/lib/src/generated/engine.dart
@@ -10532,7 +10532,7 @@ class GenerateDartErrorsTask extends AnalysisTask {
     TimeCounter_TimeCounterHandle timeCounter = PerformanceStatistics.errors.start();
     try {
       RecordingErrorListener errorListener = new RecordingErrorListener();
-      ErrorReporter errorReporter = new ErrorReporter(errorListener, source);
+      ErrorReporter errorReporter = new ErrorReporterImpl(errorListener, source);
       TypeProvider typeProvider = context.typeProvider;
       //
       // Validate the directives
@@ -14585,7 +14585,7 @@ class ResolveDartUnitTask extends AnalysisTask {
     //
     TimeCounter_TimeCounterHandle counterHandleErrors = PerformanceStatistics.errors.start();
     try {
-      ErrorReporter errorReporter = new ErrorReporter(errorListener, source);
+      ErrorReporter errorReporter = new ErrorReporterImpl(errorListener, source);
       ErrorVerifier errorVerifier = new ErrorVerifier(errorReporter, _libraryElement, typeProvider, inheritanceManager);
       unit.accept(errorVerifier);
       ConstantVerifier constantVerifier = new ConstantVerifier(errorReporter, _libraryElement, typeProvider);