Ignore missing entries in patching map

pkg/analyzer/lib/src/dart/sdk/patch.dart

Index: pkg/analyzer/lib/src/dart/sdk/patch.dart
diff --git a/pkg/analyzer/lib/src/dart/sdk/patch.dart b/pkg/analyzer/lib/src/dart/sdk/patch.dart
index 44f6765ddfc3be2fcfa0f36977d6bb8868cac162..ee0caa2ddcea4387e0bbc8c0a09e9f70c232d3f2 100644
--- a/pkg/analyzer/lib/src/dart/sdk/patch.dart
+++ b/pkg/analyzer/lib/src/dart/sdk/patch.dart
@@ -51,7 +51,7 @@ class SdkPatcher {
       _allowNewPublicNames = libraryName == '_internal';
     }
     // Prepare the patch files to apply.
-    List<String> patchPaths = allPatchPaths[libraryUriStr];
+    List<String> patchPaths = allPatchPaths[libraryUriStr] ?? const <String> [];
 
     for (String path in patchPaths) {
       File patchFile = resourceProvider.getFile(path);