Don't use Source to compute absolute URI in _getLibraryByRelativeUri().

It does not matter usually, unless we live in the environment where
actual Source resolution is very expensive. This makes resynthsis
about 10% faster in this case.

[email protected]
BUG=

Review URL: https://codereview.chromium.org/2386363002 .

Author: scheglov

pkg/analyzer/lib/src/summary/resynthesize.dart

@@ -19,8 +19,10 @@ import 'package:analyzer/src/generated/resolver.dart';
 import 'package:analyzer/src/generated/source_io.dart';
 import 'package:analyzer/src/generated/testing/ast_factory.dart';
 import 'package:analyzer/src/generated/testing/token_factory.dart';
+import 'package:analyzer/src/generated/utilities_dart.dart';
 import 'package:analyzer/src/summary/format.dart';
 import 'package:analyzer/src/summary/idl.dart';
+import 'package:analyzer/src/util/fast_uri.dart';
 
 /**
  * Implementation of [ElementResynthesizer] used when resynthesizing an element
@@ -1160,9 +1162,8 @@ class _LibraryResynthesizerContext implements LibraryResynthesizerContext {
   }
 
   LibraryElementHandle _getLibraryByRelativeUri(String depUri) {
-    String absoluteUri = resynthesizer.summaryResynthesizer.sourceFactory
-        .resolveUri(resynthesizer.librarySource, depUri)
-        .uri
+    String absoluteUri = resolveRelativeUri(
+            resynthesizer.librarySource.uri, FastUri.parse(depUri))
         .toString();
     return new LibraryElementHandle(resynthesizer.summaryResynthesizer,
         new ElementLocationImpl.con3(<String>[absoluteUri]));