allow dart:* libs to implement otherwise disallowed types (e.g. int)

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

Index: pkg/analyzer/lib/src/generated/error_verifier.dart
diff --git a/pkg/analyzer/lib/src/generated/error_verifier.dart b/pkg/analyzer/lib/src/generated/error_verifier.dart
index 89db77bf8ffc4e6aaf889bc6d63d2e49cc6a9220..a165b228ef78833a01d42067f355fe5b85e219f5 100644
--- a/pkg/analyzer/lib/src/generated/error_verifier.dart
+++ b/pkg/analyzer/lib/src/generated/error_verifier.dart
@@ -3199,6 +3199,11 @@ class ErrorVerifier extends RecursiveAstVisitor<Object> {
     if (typeName.isSynthetic) {
       return false;
     }
+    // The SDK implementation may implement disallowed types. For example,
+    // JSNumber in dart2js and _Smi in Dart VM both implement int.
+    if (_currentLibrary.source.isInSystemLibrary) {
+      return false;
+    }
     DartType superType = typeName.type;
     for (InterfaceType disallowedType
         in _DISALLOWED_TYPES_TO_EXTEND_OR_IMPLEMENT) {