Suppress scanner errors for non-existent URIs.

pkg/analyzer/lib/src/task/dart.dart

Index: pkg/analyzer/lib/src/task/dart.dart
diff --git a/pkg/analyzer/lib/src/task/dart.dart b/pkg/analyzer/lib/src/task/dart.dart
index aa910b20e2ef1f130a78e96ece9ecb0b044ef503..eb2c9213fff8edd9a1a9530df1218f7f83a714ad 100644
--- a/pkg/analyzer/lib/src/task/dart.dart
+++ b/pkg/analyzer/lib/src/task/dart.dart
@@ -3812,8 +3812,10 @@ class ScanDartTask extends SourceBasedAnalysisTask {
           message = exception.toString();
         }
       }
-      errorListener.onError(new AnalysisError(
-          source, 0, 0, ScannerErrorCode.UNABLE_GET_CONTENT, [message]));
+      if (source.exists()) {
+        errorListener.onError(new AnalysisError(
+            source, 0, 0, ScannerErrorCode.UNABLE_GET_CONTENT, [message]));
+      }
     }
     if (target is DartScript) {
       DartScript script = target;