Validate that int values that we write to summaries are >= 0.

pkg/analyzer/lib/src/summary/summarize_elements.dart

Index: pkg/analyzer/lib/src/summary/summarize_elements.dart
diff --git a/pkg/analyzer/lib/src/summary/summarize_elements.dart b/pkg/analyzer/lib/src/summary/summarize_elements.dart
index 0450a6b0f055574f46af471dc2bbea8f737290a9..f19410b2d4aaa24f778856058594a15e75de402b 100644
--- a/pkg/analyzer/lib/src/summary/summarize_elements.dart
+++ b/pkg/analyzer/lib/src/summary/summarize_elements.dart
@@ -506,7 +506,6 @@ class _CompilationUnitSerializer {
   UnlinkedImportBuilder serializeImport(ImportElement importElement) {
     UnlinkedImportBuilder b = new UnlinkedImportBuilder();
     b.isDeferred = importElement.isDeferred;
-    b.offset = importElement.nameOffset;
     b.combinators = importElement.combinators.map(serializeCombinator).toList();
     if (importElement.prefix != null) {
       b.prefixReference = serializePrefix(importElement.prefix);
@@ -515,6 +514,7 @@ class _CompilationUnitSerializer {
     if (importElement.isSynthetic) {
       b.isImplicit = true;
     } else {
+      b.offset = importElement.nameOffset;
       b.uri = importElement.uri;
       b.uriOffset = importElement.uriOffset;
       b.uriEnd = importElement.uriEnd;