Suppress scanner errors for non-existent URIs.

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

 // Copyright (c) 2014, 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.
 
 library engine;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:math' as math;
 
@@ skipping 8755 matching lines @@
   @override
   void internalPerform() {
     _complete = true;
     try {
       TimestampedData<String> data = context.getContents(source);
       _content = data.data;
       _modificationTime = data.modificationTime;
       AnalysisEngine.instance.instrumentationService
           .logFileRead(source.fullName, _modificationTime, _content);
     } catch (exception, stackTrace) {
-      errors.add(new AnalysisError(
-          source, 0, 0, ScannerErrorCode.UNABLE_GET_CONTENT, [exception]));
+      if (source.exists()) {
+        errors.add(new AnalysisError(
+            source, 0, 0, ScannerErrorCode.UNABLE_GET_CONTENT, [exception]));
+      }
       throw new AnalysisException("Could not get contents of $source",
           new CaughtException(exception, stackTrace));
     }
   }
 }
 
 /**
  * The information cached by an analysis context about an individual HTML file.
  */
 class HtmlEntry extends SourceEntry {
@@ skipping 3263 matching lines @@
     PendingFuture pendingFuture =
         new PendingFuture<T>(_context, source, computeValue);
     if (!pendingFuture.evaluate(sourceEntry)) {
       _context._pendingFutureSources
           .putIfAbsent(source, () => <PendingFuture>[])
           .add(pendingFuture);
     }
     return pendingFuture.future;
   }
 }