Fix ExplicitSourceResolver's handling of missing files.

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

Index: pkg/analyzer/lib/src/generated/source.dart
diff --git a/pkg/analyzer/lib/src/generated/source.dart b/pkg/analyzer/lib/src/generated/source.dart
index 65e8f953af24d826d5aa0e57d648f2480745a3b1..fefdc7fe62da8a23fb9dcf4d1c41927c202c090d 100644
--- a/pkg/analyzer/lib/src/generated/source.dart
+++ b/pkg/analyzer/lib/src/generated/source.dart
@@ -14,7 +14,8 @@ 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;
@@ -895,6 +896,20 @@ class UriKind extends Enum<UriKind> {
     }
     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;
+  }
 }
 
 /**