Fix for explicit dart:core import.

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 e30ed0ccf1d5e2c9b8ada4d4e43b7d0500844ef0..04529717d7faf8ca74d53c29151be749dc797c97 100644
--- a/pkg/analyzer/lib/src/task/dart.dart
+++ b/pkg/analyzer/lib/src/task/dart.dart
@@ -859,6 +859,9 @@ class BuildDirectiveElementsTask extends SourceBasedAnalysisTask {
           // directive was invalid.
           LibraryElement importedLibrary = importLibraryMap[importedSource];
           if (importedLibrary != null) {
+            if (importedLibrary.isDartCore) {
+              explicitlyImportsCore = true;
+            }
             ImportElementImpl importElement =
                 new ImportElementImpl(directive.offset);
             StringLiteral uriLiteral = importDirective.uri;