Use Element's source paths instead of URIs as a key in the index.

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 47bb6205119d914e61f3b3c59dd05a6bf81528fd..4a2c29325a4aa37344cc4400bdad46ce0dbc32b6 100644
--- a/pkg/analyzer/lib/src/generated/source.dart
+++ b/pkg/analyzer/lib/src/generated/source.dart
@@ -638,8 +638,10 @@ class SourceFactory {
       // Force the creation of an escaped URI to deal with spaces, etc.
       return _internalResolveUri(containingSource, parseUriWithException(containedUri));
     } catch (exception, stackTrace) {
+      String containingFullName =
+          containingSource != null ? containingSource.fullName : '<null>';
       AnalysisEngine.instance.logger.logError(
-          "Could not resolve URI ($containedUri) relative to source (${containingSource.fullName})",
+          "Could not resolve URI ($containedUri) relative to source ($containingFullName)",
           new CaughtException(exception, stackTrace));
       return null;
     }