Fix ExplicitSourceResolver's handling of missing files.

pkg/analyzer/lib/src/generated/source_io.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 analyzer.src.generated.source_io;
 
 import 'dart:collection';
 
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/java_core.dart';
@@ skipping 80 matching lines @@
 
   /**
    * Construct an [ExplicitSourceResolver] based on the given [uriToFileMap].
    */
   ExplicitSourceResolver(Map<Uri, JavaFile> uriToFileMap)
       : uriToFileMap = uriToFileMap,
         pathToUriMap = _computePathToUriMap(uriToFileMap);
 
   @override
   Source resolveAbsolute(Uri uri, [Uri actualUri]) {
-    return new FileBasedSource(uriToFileMap[uri], actualUri ?? uri);
+    JavaFile file = uriToFileMap[uri];
+    actualUri ??= uri;
+    if (file == null) {
+      return new NonExistingSource(
+          uri.toString(), actualUri, UriKind.fromScheme(actualUri.scheme));
+    } else {
+      return new FileBasedSource(file, actualUri);
+    }
   }
 
   @override
   Uri restoreAbsolute(Source source) {
     return pathToUriMap[source.fullName];
   }
 
   /**
    * Build the inverse mapping of [uriToSourceMap].
    */
@@ skipping 110 matching lines @@
 
   @override
   int get modificationStamp => file.lastModified();
 
   @override
   String get shortName => file.getName();
 
   @override
   UriKind get uriKind {
     String scheme = uri.scheme;
-    if (scheme == PackageUriResolver.PACKAGE_SCHEME) {
-      return UriKind.PACKAGE_URI;
-    } else if (scheme == DartUriResolver.DART_SCHEME) {
-      return UriKind.DART_URI;
-    } else if (scheme == FileUriResolver.FILE_SCHEME) {
-      return UriKind.FILE_URI;
-    }
-    return UriKind.FILE_URI;
+    return UriKind.fromScheme(scheme);
   }
 
   @override
   bool operator ==(Object object) =>
       object is FileBasedSource && id == object.id;
 
   @override
   bool exists() => file.isFile();
 
   @override
@@ skipping 303 matching lines @@
   }
 
   /**
    * Return `true` if the given URI is a `file` URI.
    *
    * @param uri the URI being tested
    * @return `true` if the given URI is a `file` URI
    */
   static bool isFileUri(Uri uri) => uri.scheme == FILE_SCHEME;
 }