Fix ExplicitSourceResolver's handling of missing files.

pkg/analyzer/lib/src/generated/source.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;
 
 import 'dart:collection';
 import "dart:math" as math;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/src/context/source.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/java_core.dart';
 import 'package:analyzer/src/generated/java_engine.dart';
 import 'package:analyzer/src/generated/java_io.dart' show JavaFile;
 import 'package:analyzer/src/generated/sdk.dart' show DartSdk;
-import 'package:analyzer/src/generated/source_io.dart' show FileBasedSource;
+import 'package:analyzer/src/generated/source_io.dart'
+    show FileBasedSource, FileUriResolver, PackageUriResolver;
 import 'package:analyzer/task/model.dart';
 import 'package:package_config/packages.dart';
 import 'package:path/path.dart' as pathos;
 
 /**
  * A function that is used to visit [ContentCache] entries.
  */
 typedef void ContentCacheVisitor(String fullPath, int stamp, String contents);
 
 /**
@@ skipping 860 matching lines @@
         return DART_URI;
       } else if (encoding == 0x66) {
         return FILE_URI;
       } else if (encoding == 0x70) {
         return PACKAGE_URI;
       }
       break;
     }
     return null;
   }
+
+  /**
+   * Return the URI kind corresponding to the given scheme string.
+   */
+  static UriKind fromScheme(String 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;
+  }
 }
 
 /**
  * The abstract class `UriResolver` defines the behavior of objects that are used to resolve
  * URI's for a source factory. Subclasses of this class are expected to resolve a single scheme of
  * absolute URI.
  */
 abstract class UriResolver {
   /**
    * Resolve the given absolute URI. Return a [Source] representing the file to which
    * it was resolved, whether or not the resulting source exists, or `null` if it could not be
    * resolved because the URI is invalid.
    *
    * @param uri the URI to be resolved
    * @param actualUri the actual uri for this source -- if `null`, the value of [uri] will be used
    * @return a [Source] representing the file to which given URI was resolved
    */
   Source resolveAbsolute(Uri uri, [Uri actualUri]);
 
   /**
    * Return an absolute URI that represents the given [source], or `null` if a
    * valid URI cannot be computed.
    *
    * The computation should be based solely on [source.fullName].
    */
   Uri restoreAbsolute(Source source) => null;
 }