Fix the test to determine whether a library is in the SDK

Change-Id: I711daae49f27ca21fb4adce3c10ccb1bfc9d72a8
Reviewed-on: https://dart-review.googlesource.com/48760
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

Author: bwilkerson

pkg/analyzer/lib/src/dart/element/element.dart

@@ -6618,8 +6618,13 @@ class LibraryElementImpl extends ElementImpl implements LibraryElement {
   bool get isDartCore => name == "dart.core";
 
   @override
-  bool get isInSdk =>
-      StringUtilities.startsWith5(name, 0, 0x64, 0x61, 0x72, 0x74, 0x2E);
+  bool get isInSdk {
+    Uri uri = definingCompilationUnit.source?.uri;
+    if (uri != null) {
+      return DartUriResolver.isDartUri(uri);
+    }
+    return false;
+  }
 
   /**
    * Return `true` if the receiver directly or indirectly imports the