Fix a type annotation in the parser to make it accept non-literal strings as part URIs in checked m…

sdk/lib/_internal/compiler/implementation/scanner/listener.dart

Index: sdk/lib/_internal/compiler/implementation/scanner/listener.dart
diff --git a/sdk/lib/_internal/compiler/implementation/scanner/listener.dart b/sdk/lib/_internal/compiler/implementation/scanner/listener.dart
index 783be1ce0aab47f07f517ba3a52a39943d80c74d..27a9b0a18ecff0bb3db9e7645dde5f4a52aa0f91 100644
--- a/sdk/lib/_internal/compiler/implementation/scanner/listener.dart
+++ b/sdk/lib/_internal/compiler/implementation/scanner/listener.dart
@@ -641,7 +641,7 @@ class ElementListener extends Listener {
     return result;
   }
 
-  LiteralString popLiteralString() {
+  StringNode popLiteralString() {
     StringNode node = popNode();
     // TODO(lrn): Handle interpolations in script tags.
     if (node.isInterpolation) {
@@ -703,7 +703,7 @@ class ElementListener extends Listener {
   }
 
   void endPart(Token partKeyword, Token semicolon) {
-    LiteralString uri = popLiteralString();
+    StringNode uri = popLiteralString();
     addLibraryTag(new Part(partKeyword, uri));
   }