Fix FastUri.resolveUri to handle cases like "/a.dart".

pkg/analyzer/lib/src/util/fast_uri.dart

Index: pkg/analyzer/lib/src/util/fast_uri.dart
diff --git a/pkg/analyzer/lib/src/util/fast_uri.dart b/pkg/analyzer/lib/src/util/fast_uri.dart
index 40681bdaae7ca21ecad90589b55ef8e265b8316c..691a51df306ffcdb02e1068e28cdcb2bc1e1b861 100644
--- a/pkg/analyzer/lib/src/util/fast_uri.dart
+++ b/pkg/analyzer/lib/src/util/fast_uri.dart
@@ -183,7 +183,8 @@ class FastUri implements Uri {
     }
     if (refPath.startsWith('../') ||
         refPath.contains('/../') ||
-        refPath.contains('/./')) {
+        refPath.contains('/./') ||
+        refPath.startsWith('/')) {
       Uri slowResult = _fallbackUri.resolveUri(reference);
       return FastUri.parse(slowResult.toString());
     }