Improve failure mode when there is no SDK (issue 25528)

pkg/analyzer/lib/src/task/dart.dart

Index: pkg/analyzer/lib/src/task/dart.dart
diff --git a/pkg/analyzer/lib/src/task/dart.dart b/pkg/analyzer/lib/src/task/dart.dart
index 7c112560219d79336a2650e87d5d18506e2959a8..872e2af69b25ea27997764ca1a719206ce7156dd 100644
--- a/pkg/analyzer/lib/src/task/dart.dart
+++ b/pkg/analyzer/lib/src/task/dart.dart
@@ -3551,6 +3551,16 @@ class ParseDartTask extends SourceBasedAnalysisTask {
     HashSet<Source> importedSourceSet =
         new HashSet.from(explicitlyImportedSourceSet);
     Source coreLibrarySource = context.sourceFactory.forUri(DartSdk.DART_CORE);
+    if (coreLibrarySource == null) {
+      String message;
+      DartSdk sdk = context.sourceFactory.dartSdk;
+      if (sdk == null) {
+        message = 'Could not resolve "dart:core": SDK not defined';
+      } else {
+        message = 'Could not resolve "dart:core": SDK incorrectly configured';
+      }
+      throw new AnalysisException(message);
+    }
     importedSourceSet.add(coreLibrarySource);
     //
     // Compute kind.