Check that script tag has at least src or inlined text, add unrelated test

pkg/polymer/lib/src/build/linter.dart

Index: pkg/polymer/lib/src/build/linter.dart
diff --git a/pkg/polymer/lib/src/build/linter.dart b/pkg/polymer/lib/src/build/linter.dart
index c1127eeec9c61a40849b39324894bbcdb695400e..9e683f422e0db64286ce47eafce6d51ec22dfebc 100644
--- a/pkg/polymer/lib/src/build/linter.dart
+++ b/pkg/polymer/lib/src/build/linter.dart
@@ -287,7 +287,14 @@ class _LinterVisitor extends TreeVisitor {
       _dartTagSeen = true;
     }
 
-    if (src == null) return;
+    var isEmpty = node.innerHtml.trim() == '';
+
+    if (src == null) {
+      if (isDart && isEmpty) {
+        _logger.warning('script tag seems empty.', span: node.sourceSpan);
+      }
+      return;
+    }
 
     if (src.endsWith('.dart') && !isDart) {
       _logger.warning('Wrong script type, expected type="application/dart".',
@@ -301,7 +308,7 @@ class _LinterVisitor extends TreeVisitor {
       return;
     }
 
-    if (node.innerHtml.trim() != '') {
+    if (!isEmpty) {
       _logger.warning('script tag has "src" attribute and also has script '
           'text.', span: node.sourceSpan);
     }