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

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

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.src.task.dart;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
@@ skipping 3533 matching lines @@
           }
         }
       }
     }
     //
     // Always include "dart:core" source.
     //
     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.
     //
     SourceKind sourceKind = SourceKind.LIBRARY;
     if (modificationTime == -1) {
       sourceKind = SourceKind.UNKNOWN;
     } else if (hasPartOfDirective && !hasNonPartOfDirective) {
       sourceKind = SourceKind.PART;
     }
@@ skipping 1882 matching lines @@
 
   @override
   bool moveNext() {
     if (_newSources.isEmpty) {
       return false;
     }
     currentTarget = _newSources.removeLast();
     return true;
   }
 }